In This Article Sleep and Dreaming

Psychology Sleep and Dreaming
by
Edward F. Pace-Schott, Bethany J. Jones
  • LAST REVIEWED: 12 May 2017
  • LAST MODIFIED: 10 March 2015
  • DOI: 10.1093/obo/9780199828340-0163

Introduction

Contemporary study of sleep constitutes one of the most interdisciplinary and unifying of topics in psychology and neuroscience, with investigation focused at all levels of organization from the genetics and molecular physiology of sleep to public policy studies focused on sleep behavior in society as a whole. Historically, sleep researchers have quickly leveraged biomedical technologies as they emerged to provide greater and greater understanding of the controlling mechanisms and phenomenology of sleep. Despite such advances, sleep remains to this day one of the most mysterious and controversial topics in neuroscience, and there still is no generally agreed upon “function” for this behavioral state that occupies one-third of our lives. Like the other neurosciences, sleep science has seen a remarkable increase in discoveries during the past several decades. Sleep science exemplifies the broad category of translational biomedical science, whereby the parallel development of human and animal experimentation led to the emergence of the field of sleep medicine.

General Overviews

There are an immense number of books for the general readership written on sleep science and dreaming. These vary in amount of scientific detail, and most are targeted to an adult or older adolescent audience. If one’s aim is to learn the scientific facts about sleep, it is important to be rather selective since there is a lot of entertaining, but highly speculative, writing that is promoted online or in bookstores, particularly on the topic of dreaming. Toward this end, suggested here are some introductions to the field of sleep and dreaming by investigators and clinicians who have also published extensively in the scientific literature. Two of these sources, Sleep (Hobson 1989) and The Dreaming Brain (Hobson 1988) are written by J. Allan Hobson, one of the field’s leading scientists who made important discoveries on the brain bases of sleep in animals and later became a leading expert on the neurobiology of dreaming. William Dement (Dement and Vaughan 2001) is one of the principal founders of the field of sleep medicine. Lawrence J. Epstein (Epstein and Mardon 2006) is a leading sleep physician and educator in sleep medicine. Carlos Schenck (Schenck 2007) is an international authority on the unusual wake-like behaviors that can happen during sleep (the parasomnias). Associations with a mission to disseminate scientific research provide suggested readings on sleep (National Sleep Foundation) and on dreams (International Association for the Study of Dreams) on their websites. Deirdre Barrett and Patrick McNamara are leading researchers and writers on dreaming and provide both an extensive scientific compilation (Barrett and McNamara 2007) and books for the general public (Barrett 2001). In addition to books, there now exist a wide variety of formats in which to explore this field including e-books, other electronic media like DVDs, such as Heller 2013, as well as archived podcasts and public television programs.

  • Barrett, D. 2001. The committee of sleep: How artists, scientists, and athletes use dreams for creative problem-solving—and how you can too. New York: Crown.

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    A fascinating volume detailing the many ways in which dreams have contributed to society and culture, written by the editor of the journal Dreaming—the official publication of the International Association for the Study of Dreaming. Dr. Barrett provides authoritative accounts of the many well-known as well as more obscure discoveries, artistic creations and problems solved during sleep and dreaming.

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    • Barrett D., and P. McNamara, eds. 2007. The new science of dreaming. 3 vols. Westport, CT: Praeger, Greenwood.

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      A three-volume set that discusses recent research on the biology (Vol. 1), content (Vol. 2) and societal effects (Vol. 3) of dreaming. Different chapters are written by a wide variety of leading investigators who delve into the many different ways in which the study of dreaming can be approached. Although intended also for a scientific audience of sleep specialists and dream psychologists, many of the chapters, especially those in Volumes 2 and 3, are fully accessible to general readership.

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      • Books on Dreams and Dreaming. International Association for the Study of Dreams.

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        This website provides a list of books on the scientific, clinical, as well as popular explorations of dreams and dreaming.

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        • Dement, W. C., and C. C. Vaughan. 2001. The promise of sleep: A pioneer in sleep medicine explores the vital connection between health, happiness, and a good night’s sleep. London: Pan.

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          Dement and Vaughan give both an overview of sleep science and medicine as well as a wealth of practical advice on the connections of sleep with physical and mental health.

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          • Epstein, L. J., and S. Mardon. 2006. The Harvard Medical School guide to a good night’s sleep. New York: McGraw-Hill.

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            Epstein and Mardon give advice on “sleep hygiene”—ways in which to maximize one’s quality of sleep—as well as practical information on insomnia and other sleep disorders.

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            • Heller, C. H. 2013. Secrets of sleep science: From dreams to disorders. DVD. Chantilly, VA: The Great Courses Teaching Company.

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              Professor H. Craig Heller of Stanford University is an expert on circadian rhythms. This presentation covers twenty-four topics ranging from neuroanatomy to seasonal rhythms and the disorders of sleep that are treated by sleep physicians.

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              • Hobson, J. A. 1988. The dreaming brain. New York: Basic Books.

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                An engaging historical account of dream research and the discovery of the neural mechanisms of sleep architecture (the pattern of sleep stages across the night). This volume contains an extended exposition of the first biological theory of dreaming, the Activation-Synthesis Hypothesis (see Heller 2013), and applies it to a lengthy dream journal complete with the dreamer’s illustrations of his dreams.

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                • Hobson, J. A. 1989. Sleep. New York: Scientific American Library.

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                  A concise summary of sleep science up to the late 1980s, much of which remains accurate to this day. An abundantly illustrated volume, Sleep gives a historical overview of the physiological discoveries leading to modern sleep science, covers basics of human sleep physiology as well as sleep across the human lifespan and in the animal kingdom, and gives an exposition of the first biological theory of dreaming, the Activation-Synthesis Hypothesis. This hypothesis posits that self-stimulation of the forebrain by the brainstem during REM sleep is interpreted by the sensory cortex as incoming sensory information leading to hallucinations (mainly visual) that the brain then tries to knit together into a coherent narrative.

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                  • Schenck, C. 2007. Sleep: The mysteries, the problems, and the solutions. New York: Penguin.

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                    An introduction to sleep medicine, with a focus on the parasomnias which include sleep-walking, acting out of dreams (REM Behavior Disorder), night terrors, nightmares, and more recently discovered phenomena such as sleep eating.

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                    • Sleep Books. National Sleep Foundation (NSF).

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                      Provides a list of books on sleep and health useful to anyone interested in learning more about topics such as sleep disorders, the effects of insufficient sleep on driving, children’s sleep, and improving one’s quality of sleep.

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                      Textbooks and Reference Volumes

                      As the fields of sleep medicine, psychology and neuroscience have rapidly expanded, the need for textbooks and reference volumes has increased both for training sleep physicians and technology specialists and for students of psychology and neuroscience. A representative listing of such volumes will be presented. Also listed will be just a few representative samples of the rapidly increasing numbers of reference works addressing specific topics in sleep science and medicine, such as on specific neuromodulatory substances regulating sleep (e.g., Monti, et al. 2008a and Monti, et al. 2008b cited under Specialized Reference Volumes on the Neurobiology of Sleep).

                      General Introductions to Sleep Science

                      The sources cited below are textbook chapters (Pace-Schott and Hobson 2012) or volumes of readings (Sleep Research Society 2009; Stickgold and Walker 2009) intended to provide the reader with a concise scientific overview of the entire field of sleep science or sleep medicine (Dement 2005).

                      • Dement, W. C. 2005. History of sleep medicine. Neurologic Clinics 23:945–965.

                        DOI: 10.1016/j.ncl.2005.07.001Save Citation »Export Citation »E-mail Citation »

                        An engaging account for the clinician on the historical development of sleep medicine by one of its founders.

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                        • Pace-Schott, E. F., and J. A. Hobson. 2012. The neurobiology of sleep and dreaming. In Fundamental neuroscience. 4th ed. Edited by L. R. Squire, D. K. Berg, F. E. Bloom, S. du Lac, A. Ghosh, and N. C. Spitzer, 847–869. New York: Academic Press.

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                          Many neuroscience and psychology textbooks provide chapters on sleep and dreaming. This chapter is one example of such that provides an overview of sleep architecture (the pattern of sleep stages occurring across the night), cellular physiology, and the neuromodulators (brain chemicals) of sleep, circadian rhythms, systems neuroscience of sleep, dreaming as well as the phylogeny, ontogeny, disorders, and functions of sleep.

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                          • Sleep Research Society. 2009. SRS Basics of Sleep Guide. 2d ed. Darien, IL: Sleep Research Society.

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                            An indispensable handbook and introduction to sleep science that provides concise summaries of most major topics in the Principles and Practice of Sleep Medicine, Fourth Edition. Major subdivisions include: “Section 1: The Evolution and Demography of Sleep; Section 2: Life Cycles in Sleep; Section 3: Neurobiology, Neurochemistry and Biochemistry of Sleep; Section 4: Sleep Disorders; Section 5: Genetics; Section 6: Physiology in Sleep; Section 7: Chronobiology; Section 8: Sleep Deprivation.”

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                            • Stickgold, R., and M. P. Walker, eds. 2009. The neuroscience of sleep. Philadelphia: Elsevier.

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                              Key chapters on sleep neuroscience from the Encyclopedia of Neuroscience, edited by Larry R. Squire, et al. (Philadelphia: Elsevier, 2008). The editors of this text are two of the leading investigators on sleep-dependent memory consolidation. Available online.

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                              Standard References on Sleep Scoring and Sleep Medicine

                              The following resources include the original (Rechtschaffen and Kales 1968) and recently revised (Iber, et al. 2007) sleep stage scoring manuals as well as the regularly updated major sleep medicine reference volume from the American Academy of Sleep Medicine (Kryger, et al. 2011) and the standard diagnostic manual for sleep disorders (American Academy of Sleep Medicine 2014). Sleep scoring refers to the identification of specific sleep stages using polysomnography (PSG), which measures brain activity (electroencephalography/EEG), eye movements (electrooculography/EOG), and muscle tone (electromyography/EMG). Sleep consists of two distinct behavioral sub-states—rapid eye movement (REM) and non-REM (NREM) sleep—that alternate with a period of about 90 minutes in humans. NREM sleep contains three stages and predominates early in the night, whereas REM sleep occupies a greater proportion of each cycle later in the night. The PSG features used to distinguish the three phases of NREM from each other and from REM include the predominant frequencies of EEG rhythms (or “oscillations”), the degree of muscle tone in the EMG, and activity of the eyes in the EOG. Major categories of sleep disorders include insomnias, which are characterized by problems initiating and/or maintaining sleep; hypersomnias (e.g., narcolepsy), characterized by excessive daytime sleepiness; sleep-related breathing disorders, such as sleep apnea; parasomnias, characterized by abnormal behaviors during sleep (e.g., sleep walking, nightmares, and bedwetting); circadian rhythm sleep disorders, which involve disruption in the timing of sleep; and sleep-related movement disorders, such as restless legs syndrome.

                              • American Academy of Sleep Medicine. 2014. International Classification of Sleep Disorders. 3d ed. Darien, IL: American Academy of Sleep Medicine.

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                                The most recent edition of the definitive AASM text (ICSD-3) providing classification, diagnostic criteria, and codes for sleep disorders including insomnias (difficulty sleeping) and hypersomnias (excessive sleep), sleep-related breathing disorders (such as sleep apnea), circadian rhythm disorders (disorders of the timing of sleep), parasomnias (wake-like behaviors occurring during sleep) and movement disorders in sleep (such as periodic limb movement disorder). This volume also provides cross-reference to the codes for these same disorders in the ninth and tenth editions of the International Classification of Diseases.

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                                • Iber, C., et al. 2007. The AASM manual for the scoring of sleep and associated events: Rules, terminology and technical specification. Westchester, IL: American Academy of Sleep Medicine.

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                                  The aforementioned update, by the American Academy of Sleep Medicine (AASM), of the gold standard Rechtschaffen and Kales manual. Changes made were minor and, due to its continuity and comparability with past literature and scoring software, this newer scoring manual has been widely accepted by both the clinical and sleep-research communities. Introduced were new specific staging criteria and stage-naming conventions.

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                                  • Kryger, M. H., T. Roth, and W. C. Dement, eds. 2011. Principles and practice of sleep medicine. 5th ed. Philadelphia: Elsevier.

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                                    This is the definitive reference on sleep medicine. This volume is written under the auspices of the AASM, and new editions are published approximately every five years. The leading authorities on sleep medicine and science author each of its chapters. The major subheadings are “Principles of Sleep Medicine” covering the basic foci of sleep research (e.g., neuronal systems control, physiology, genetics) and “Practice of Sleep Medicine” covering diagnosis, pathophysiology and treatment of sleep disorders.

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                                    • Rechtschaffen, A., and A. Kales. 1968. A manual of standardized terminology techniques and scoring system for sleep stages of human subjects. Los Angeles: Brain Information Service/Brain Research Institute, Univ. of California at Los Angeles.

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                                      Until 2007, this manual provided the standards for scoring sleep stages based on polysomnography (i.e., EEG, EOG, and EMG). Written by two of the founders of sleep science and medicine, this set of rules formed the common basis of tens of thousands of publications worldwide until it was revised in 2007, and even then, the basic rules remain in the revision.

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                                      Specialized Reference Volumes on the Neurobiology of Sleep

                                      These sources provide more detailed, comprehensive overviews of a specific sleep stage (Mallick, et al. 2011), sleep neurobiology (Steriade and McCarley 2005; Lydic and Baghdoyan 1999; Monti, et al. 2008a) or a specific neurotransmitter (i.e., brain chemicals by which neurons communicate) involved in sleep regulation (Monti, et al. 2008b). It is important to remember that this is just a small sample of an immense and growing set of books on the different aspects of sleep available as hard copy or e-book.

                                      • Lydic, R., and H. A. Baghdoyan, eds. 1999. Handbook of behavioral state control: Molecular and cellular mechanisms. Boca Raton, FL: CRC Press.

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                                        An older volume summarizing key cellular and molecular mechanisms of sleep and behavioral state control (control of different states of consciousness such as sleep and wake) circa the late 1990s.

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                                        • Mallick, B. N., S. R. Pandi-Permual, R. W. McCarley, and A. R. Morrison, eds. 2011. Rapid eye movement sleep: Regulation and function. Cambridge, UK: Cambridge Univ. Press.

                                          DOI: 10.1017/CBO9780511921179Save Citation »Export Citation »E-mail Citation »

                                          The second edition of this definitive text on REM sleep with chapters by over forty leading authorities on specific aspects of this unique behavioral state. Major sections include “REM Sleep as a Unique Arousal State – Historical Context,” “General Biology,” “Neuronal Regulation,” “Neuroanatomy and Neurochemistry,” “REM Sleep: Functional Significance,” “Disturbance in REM Sleep Generating Mechanism.”

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                                          • Monti, J. M., S. R. Pandi-Perumal, B. Jacobs, and D. Nutt, eds. 2008b. Serotonin and sleep: Molecular, functional and clinical aspects. Basel, Switzerland: Birkhauser-Verlag.

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                                            A reference specifically on the many roles of the neuromodulator serotonin in the control of sleep/wake and REM/NREM sleep cycles.

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                                            • Monti, J. M., S. R. Pandi-Perumal, and C. M. Sinton, eds. 2008a. Neurochemistry of sleep and wakefulness. Cambridge, UK: Cambridge Univ. Press.

                                              DOI: 10.1017/CBO9780511541674Save Citation »Export Citation »E-mail Citation »

                                              A more up-to-date volume covering the same basic topics as Lydic and Baghdoyan 1999.

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                                              • Steriade, M., and R. W. McCarley. 2005. Brain Control of Wakefulness and Sleep. 2d ed. New York: Kluwer Academic/Plenum.

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                                                The second edition of a major reference first published in 1990 by the leading authorities on the electrophysiology (i.e., measurements of the electrical activity either within the brain of animals or at the scalp of humans) of sleep (the late Mircea Steriade) and on the neurochemistry and neuronal control of sleep (Robert McCarley).

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                                                Reference Volumes on Dreaming

                                                These references compile the full range of topics considered by contemporary dream researchers (Barrett and McNamara 2007, McNamara and Barrett 2012), diverse opinion on major scientific controversies surrounding dreams (Pace-Schott, et al. 2003), or an introduction to the evolutionary psychology of dreams (McNamara 2004).

                                                • Barrett D., and P. McNamara, eds. 2007. The New Science of Dreaming. Westport, CT: Praeger, Greenwood Press.

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                                                  A comprehensive guide to the most recent research on dreaming circa 2007 containing chapters by many of the leading dream researchers and edited by the editor of the journal Dreaming (Deirdre Barrett, Ph.D.) and Patrick McNamara, Ph.D. (an expert of the evolutionary psychology of dreams). This reference is divided into three volumes: “Volume 1: The Biology of Dreaming; Volume 2: Content, Recall, and Personality Correlates of Dreams; Volume 3: Cultural and Theoretical Perspectives on Dreaming.”

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                                                  • McNamara, P. 2004. An evolutionary psychology of sleep and dreams. Westport, CT: Praeger Publishing.

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                                                    A concise overview of major theories on the evolution of sleep and dreaming by a leading authority on the evolutionary psychology of dreaming.

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                                                    • McNamara, P., and D. Barrett, eds. 2012. Encyclopedia of sleep and dreams: The evolution, function, nature, and mysteries of slumber. 2 vols. Santa Barbara, CA: Greenwood.

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                                                      A two-volume print, e-book, and online resource covering a broad range of topics on dreaming as well as sleep in an alphabetical format. Contains 330 entries, suggested readings and primary sources as well as specific sections on dreams. Topics are drawn from sleep biology, neuroscience, medicine, and psychology as well as historical and anthropological perspectives.

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                                                      • Pace-Schott, E. F., M. Solms, M. T. Blagrove, and S. Harnad, eds. 2003. Sleep and dreaming: Scientific advances and reconsiderations. Cambridge, UK: Cambridge Univ. Press.

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                                                        Composed mostly of material from Behavioral and Brain Sciences 23(6) 2000, an issue with five target articles, eighty-three commentaries on these articles by prominent sleep and dream researchers, responses to these commentaries, and an update on sleep and dreaming circa 2003.

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                                                        Textbooks on Sleep Medicine

                                                        With the expansion of clinical sleep centers and provision of physician board certification in sleep medicine by the American Academy of Sleep Medicine (AASM), a large number of textbooks on sleep medicine have recently been published. These volumes are of interest to physicians specializing in neurology, psychiatry, pulmonology, and otolaryngology and well as psychologists and polysomnographic technologists. Texts in many subspecialties of sleep medicine are now widely available. Representative examples are behavioral sleep medicine (Perlis, et al. 2010, cited under Resources Dealing with Specific Sleep Disorders and Their Treatments), pediatric sleep medicine (Sheldon, et al. 2005, cited under General Textbooks in Sleep Medicine), circadian rhythm disorders (Wright 2009, cited under Resources Dealing with Specific Sleep Disorders and Their Treatments), and pulmonology (Fairbanks, et al. 2003 and Lurie 2013, both cited under Resources Dealing with Specific Sleep Disorders and Their Treatments). The AASM provides an online listing of publications that can be purchased by clinicians and researchers interested in specific areas of sleep medicine. A number of textbooks specifically for polysomnographic technicians also exist such as Butkov and Lee-Chiong 2007 and Spriggs 2008 (both cited under Technical References for the Sleep Technologist or Physician).

                                                        General Textbooks in Sleep Medicine

                                                        These resources are intended for the sleep medicine specialist or general practitioner wishing an overview of sleep medicine or to review for board exam (Lee-Chiong 2008).

                                                        • Berry, R. B. 2011. Fundamentals of sleep medicine. Philadelphia: Elsevier/Saunders.

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                                                          This text provides a concise overview of topics in sleep medicine for both the sleep clinician and the layperson.

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                                                          • Chokroverty, S., and R. J. Thomas. 2005 Atlas of sleep medicine. Philadelphia: Elsevier.

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                                                            A more comprehensive overview of topics in sleep medicine for the sleep physician and technologist. This volume provides traces of the different channels used in polysomnography (EEG, EOG, EMG), illustrating sleep stages and symptoms of sleep disorders, as well as information on conducting sleep studies.

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                                                            • Lee-Chiong, T. L. 2008. Sleep medicine: Essentials and review. New York: Oxford Univ. Press.

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                                                              This volume covers the basic knowledge areas required for board certification of sleep physicians and doubles as a quick reference to the basics of sleep medicine.

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                                                              • Sheldon, S. H., M. H. Kryger, and R. Ferber. 2005. Principles and practice of pediatric sleep medicine. Elsevier.

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                                                                This volume, written by the field’s leading clinician investigators on the treatment of childhood sleep disorders, serves as a companion to the Principles and Practice of Sleep Medicine, fifth edition, described above.

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                                                                Technical References for the Sleep Technologist or Physician

                                                                These volumes provide comprehensive information on polysomnographic techniques along with many sample tracings to illustrate specific features seen in PSG recordings.

                                                                • Butkov, N., and T. L. Lee-Chiong. 2007. Fundamentals of sleep technology. Philadelphia: Lippincott Williams and Wilkins.

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                                                                  A definitive volume on the techniques for carrying out and interpreting clinical polysomnographic studies of sleep that is endorsed by the American Association of Sleep Technologists (AAST).

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                                                                  • Spriggs, W. H. 2008. Essentials of polysomnography: A training guide and reference for sleep technicians. Boston: Jones and Bartlett.

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                                                                    A comprehensive reference for sleep laboratory technicians.

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                                                                    Resources Dealing with Specific Sleep Disorders and their Treatments

                                                                    Sleep medicine has developed to the point where textbooks and training manuals are available for specific disorders such as insomnia (Perlis, et al. 2010), circadian disorders (Wright 2009) and sleep apnea (Fairbanks, et al. 2003; Lurie 2013). Additional specialized texts can be found at the AASM website.

                                                                    • American Academy of Sleep Medicine Resource Library.

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                                                                      An online listing of resources specifically for sleep clinicians that can be purchased in hard copy or electronic forms of media.

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                                                                      • Fairbanks, D. N. F., S. A. Mickelson, and B. T. Woodson, eds. 2003. Snoring and obstructive sleep apnea. Philadelphia: Lippincott Williams & Wilkins.

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                                                                        Sleep apnea is the most frequently encountered sleep disorder and its diagnosis and treatment constitute the “bread and butter” of modern sleep disorders clinics. Snoring is frequently but not always associated with sleep apnea.

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                                                                        • Lurie, A. 2013. Obstructive sleep apnea in adults: Relationship with cardiovascular and metabolic disorders. Farmington, CT: Karger.

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                                                                          Some of the most serious consequences of untreated sleep apnea are the greatly elevated risk of hypertension and cardiovascular disease.

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                                                                          • Perlis, M. L., M. Aloia, and B. Kuhn, eds. 2010. Behavioral treatments for sleep disorders: A comprehensive primer of behavioral sleep medicine interventions. New York: Academic Press.

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                                                                            One of the most recent treatment manuals for the behavioral management of insomnia and other sleep disorders written by leading experts from the Behavioral Sleep Medicine Program at the University of Pennsylvania School of Medicine.

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                                                                            • Wright, K. P. 2009. Circadian rhythm sleep disorders. Philadelphia: Saunders.

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                                                                              An issue of Sleep Medicine Clinics, which provides periodic reviews and updates on the clinical practice of sleep medicine. This particular issue provides an up-to-date and comprehensive set of articles on circadian disorders (i.e., abnormalities in the timing of sleep/wake periods).

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                                                                              Journals on Sleep

                                                                              There are an increasing number of sleep-based journals available in digital and hard-copy library collections as well as open source journals online. Additionally, certain journals frequently publish sleep studies from clinical, psychological or cognitive neuroscience perspectives. An increasing number of open-access, peer-reviewed journals are becoming available such as The Open Sleep Journal. Here, we list several well-established periodicals that specifically focus on sleep and closely related topics.

                                                                              Journals on Dreaming

                                                                              As in the case of reference volumes, there are only a few journals devoted specifically to dreaming, although articles on dreaming occasionally appear in the Journals on Sleep, especially in older volumes. One journal devoted to dreaming (Dreaming) and three in which articles on dreaming frequently appear (Consciousness and Cognition, Sleep and Hypnosis, and Psychology of Consciousness) are included here.

                                                                              Professional Associations

                                                                              There are a number of organizations to which clinical sleep specialists and sleep researchers typically belong. The American Academy of Sleep Medicine (AASM), Sleep Research Society (SRS), and American Association of Sleep Technologists (AAST) are notable as they hold their annual meetings concurrently in the same location (Associated Professional Sleep Societies or APSS). Student memberships are available to most of these organizations allowing trainees to attend meetings at lowered cost as well as to have electronic access to their respective journals.

                                                                              Neurobiology of Sleep

                                                                              The development of the human scalp electroencephalogram (EEG) by Hans Berger in 1924 led to the discovery of rapid eye movement (REM) sleep by Aserinsky and Kleitman in 1953. In the past few decades, this has led to advanced quantitative technologies to study sleep such as EEG spectral analysis (determining how much the different frequencies of brain waves contribute to the observed scalp EEG) and EEG source localization (determining where certain frequencies originate in the brain) as well as to measurement of the brain’s magnetic activity using magnetoencephalography (MEG). Early neurophysiological studies in animals identified the networks of brain structures that produce the EEG; networks that, until the recent development of intracranial EEG (implantation of electrodes in the brain to locate sources of epileptic seizures), could only be measured at the scalp in humans. Key among these early discoveries were the ascending reticular activating system (a network of neurons in the core of the brainstem required for wakefulness) by Morruzzi and Magoun in 1949 and specific groups of neurons in the brainstem that produce specific neuromodulators by Dahlstrom and Fuxe in 1964. (Neuromodulators and neurotransmitters are the chemicals used by neurons to communicate at the synapse as well as elsewhere on the neuron cell surface.) Further discoveries by researchers such as Mircea Steriade, J. Allan Hobson, Robert McCarley, and Clifford Saper demonstrated that these systems control sleep-wake and REM-non-REM (NREM) cycles. Animal research by Steriade, David McCormick, and others also revealed thalamo-cortical oscillations (repeating firing patterns among loops of interconnected neurons in the thalamus and cortex) that produce the EEG brainwaves seen during NREM. Most recently, such studies have led to the concept of “local sleep” (the notion that certain groups of neurons may “sleep” while the rest of the brain stays “awake”). Animal research also led to discovery of the molecular machinery of twenty-four-hour circadian rhythms that exists in each cell of the “master clock” in the suprachiasmatic nucleus (SCN) in the hypothalamus (a small group of neurons that sits just above where the two optic nerves cross). Animal research also produced gene microarray technologies (testing for activity of many different genes at once) that allow search for the genetic determinants of sleep on an immense scale, and optogenetic techniques (attaching special receptors to the surface of cells that can be opened by beams of light) that can show direct causal pathways controlling sleep. In humans, positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) (the two major forms of modern functional neuroimaging) can now reveal changes in brain activity that accompany different sleep stages. Such functional neuroimaging (imaging of the brain’s activity as opposed to structural neuroimaging that looks at anatomy) can be carried out simultaneously with PSG during sleep or sequentially before and after PSG-recorded sleep to investigate the influence of sleep stages on waking cognition. Magnetic resonance spectroscopy (MRS) techniques (determining chemical composition of the brain by looking at how different chemicals respond to the magnetic field) can now also show how sleep or lack of sleep affects brain chemistry. Also, using PET, radioactive chemicals that mimic specific neurotranmitters can be injected and then visualized when they attach to specific receptors within the brain. fMRI is now also used to demonstrate functional connectivity (parts of the brain that respond to the magnetic field in synchrony) among different regions of the sleeping brain. These patterns can then be compared with animal and human postmortem neuroanatomical studies as well as with diffusion tensor imaging (DTI) of fiber tracts in the living brain. (DTI uses MRI to visualize major nerve pathways using the movement of water molecules.) With the advent of fMRI, there has emerged renewed interest in lucid dreaming (realizing that one is dreaming while dreaming) which allows the dreamer to “report out” when they are performing a fictive (imagined) behavior, the brain activity accompanying which can then be observed.

                                                                              Models and Theories of Sleep Neurobiology

                                                                              The neuroscience of sleep has benefitted greatly from heuristic models that have guided experimental design and evolved over time. Two such models are Borbély’s Two Process Model of behavioral sleep propensity—presented in Borbély 1982, Borbély and Achermann 1999—and Hobson and McCarley’s Reciprocal Interaction Model of REM-NREM oscillation—presented in Hobson, et al. 1975 and McCarley and Hobson 1975. The Two Process Model suggests that two interacting processes regulate sleep propensity. The first, Process S, is the homeostatic drive to sleep that increases with increasing time spent awake. The second (Process C) is the circadian drive to sleep which is controlled by the SCN and increases or decreases based on the specific time of day. The search for the basis of Process S has led to the identification of adenosine as a prime candidate for an endogenous somnogen (sleep-promoting substance that builds up with increasing time awake). Research on Process S has also identified spectral power of the delta-frequency (0.5–4 Hertz or cycles per second, abbreviated “Hz”) in the EEG during slow wave sleep (SWS) as the best marker of the degree of accumulated sleep need in the brain as a whole and, for “local sleep,” in specific parts of the brain. To account for transitions between REM and NREM sleep stages, the Reciprocal Interaction Theory suggested that two groups of neurons in the brainstem—those that produce monoamine neuromodulators (serotonin and norepinephrine) and those that produce acetylcholine—inhibit one another but excite themselves resulting in a regular repeating pattern of first one being most active and then the other being so. Although this original formulation of this theory has proved more complex, the principles of this model have informed later models. For example, those presented in Saper, et al. 2001; Saper, et al. 2010; Luppi, et al. 2006 and others postulate similar dynamics among brainstem neurons that produce gamma amino butyric acid (GABA)—the major inhibitory neurotransmitter of the brain. Saper’s model of reciprocal inhibition suggests that sleep-promoting and wake-promoting groups of neurons inhibit each other (reciprocal inhibition), an arrangement that, similarly to reciprocal interaction, results in first one group and then the other being active (sometimes called a “flip-flop switch”). Saper and colleagues have most recently suggested that a similar flip-flop mechanism controls the regular alternation of REM and NREM in the sleeping brain (Saper, et al. 2010).

                                                                              • Borbély, A. A. 1982. A two-process model of sleep regulation. Human Neurobiology 1:195–204.

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                                                                                Exposition of the leading theory of sleep propensity regulation. Circadian rhythms (Process C) are processes controlled by the endogenous twenty-four-hour clock, and sleep homeostasis (Process S) are those controlled by duration of prior waking. Together, they control sleep propensity at any one time.

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                                                                                • Borbély, A. A., and P. Achermann. 1999. Sleep homeostasis and models of sleep regulation. Journal of Biological Rhythms 14:557–568.

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                                                                                  An updated presentation of the Two Process Model of sleep propensity.

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                                                                                  • Hobson, J. A., R. W. McCarley, and P. W. Wyzinki. 1975. Sleep cycle oscillation: Reciprocal discharge by two brainstem neuronal groups. Science 189:55–58.

                                                                                    DOI: 10.1126/science.1094539Save Citation »Export Citation »E-mail Citation »

                                                                                    An early investigation of brainstem activity during wake, NREM sleep, and REM sleep in the cat. These investigators found opposite activity patterns in two cell groups, providing evidence for the “Reciprocal Interaction Theory” for the regular alternation of REM and NREM sleep.

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                                                                                    • Luppi, P. H., D. Gervasoni, L. Verret, et al. 2006. Paradoxical (REM) sleep genesis: The switch from an aminergic-cholinergic to a GABAergic-glutamatergic hypothesis. Journal of Physiology – Paris 100:271–283.

                                                                                      DOI: 10.1016/j.jphysparis.2007.05.006Save Citation »Export Citation »E-mail Citation »

                                                                                      Reviews evidence that the neuron groups responsible for transitions between NREM and REM sleep are different than those proposed by the original Reciprocal Interaction Theory. Presents a GABA-ergic model for the brainstem control of REM-NREM alternation.

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                                                                                      • McCarley, R. W., and J. A. Hobson. 1975. Neuronal excitability modulation over the sleep cycle: A structural and mathematical model. Science 189:58–60.

                                                                                        DOI: 10.1126/science.1135627Save Citation »Export Citation »E-mail Citation »

                                                                                        Presents the Reciprocal Interaction Theory of REM-NREM control described above.

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                                                                                        • Saper, C. B., T. C. Chou, and T. E. Scammell. 2001. The sleep switch: hypothalamic control of sleep and wakefulness. Trends in Neurosciences 24:726–731.

                                                                                          DOI: 10.1016/S0166-2236(00)02002-6Save Citation »Export Citation »E-mail Citation »

                                                                                          Reviews the neurobiology of sleep and wakefulness with a focus on neuronal groups in the hypothalamus. Proposes the “flip-flop switch” model, which posits that mutually inhibitory interactions between wake-promoting and sleep-promoting neurons control transitions between sleep and wake states (see introductory paragraph above). Includes figures.

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                                                                                          • Saper, C. B., P. M. Fuller, N. P. Pedersen, J. Lu, and T. E. Scammell. 2010. Sleep state switching. Neuron 68.6: 1023–1042.

                                                                                            DOI: 10.1016/j.neuron.2010.11.032Save Citation »Export Citation »E-mail Citation »

                                                                                            An updated review of the theory and evidence for the flip-flop switch model (reciprocal inhibition) of sleep-wake and REM-NREM control. Reviews the neural circuitry and dynamics between neuron groups underlying transitions between sleep and wake as well as between NREM and REM sleep stages.

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                                                                                            • Tononi, G., and C. Cirelli. 2006. Sleep function and synaptic homeostasis. Sleep Medicine Reviews 10:49–62.

                                                                                              DOI: 10.1016/j.smrv.2005.05.002Save Citation »Export Citation »E-mail Citation »

                                                                                              Exposition of the synaptic homeostasis model of sleep function. Posits that sleep, specifically SWS, functions to downscale the strength of synaptic connections that builds up over waking and thereby maintains “synaptic homeostasis” which prevents runaway energy costs.

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                                                                                              Reviews on the General Neurobiology of Sleep

                                                                                              Our knowledge on the neurobiology of sleep is constantly expanding. This results in published reviews frequently lacking the most up-to date information. Therefore, although the following are outstanding reviews of the field at their approximate time of publication, the student of sleep science truly wishing to keep up with this field is advised to frequently peruse the journals listed in Journals on Sleep and Journals on Dreaming and frequently perform data-base searches using key words describing his or her main areas of interest. That being said, reviews pulling disparate tracks together in one place are invaluable to achieving an overview of sleep neuroscience. The following reviews will strongly reflect the authors’ research focus and, hence, consulting several is best for a comprehensive view. For example, reviews by cellular neurophysiologists will provide detail on the subcortical networks involved in sleep-state transitions (e.g., Datta and Maclean 2007, Datta 2011), whereas reviews by clinicians will focus on medical interventions (e.g., Espana and Scammell 2011, Watson, et al. 2012).

                                                                                              • Datta, S. 2011. Cellular and chemical neuroscience of mammalian sleep. Sleep Medicine 11:431–440.

                                                                                                DOI: 10.1016/j.sleep.2010.02.002Save Citation »Export Citation »E-mail Citation »

                                                                                                Introduces the sleep-wake cycle and reviews its underlying neurobiology.

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                                                                                                • Datta, S., and R. R. Maclean. 2007. Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: Reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence. Neuroscience and Biobehavioral. Reviews 31:775–824.

                                                                                                  DOI: 10.1016/j.neubiorev.2007.02.004Save Citation »Export Citation »E-mail Citation »

                                                                                                  A comprehensive review of the neural basis of the sleep-wake cycle, including historical findings. Presents a “cellular-molecular-network model” of REM sleep regulation. Includes helpful figures.

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                                                                                                  • Espana, R. A., and T. E. Scammell. 2011. Sleep neurobiology from a clinical perspective. Sleep 34.7: 845–858.

                                                                                                    DOI: 10.5665/SLEEP.1112Save Citation »Export Citation »E-mail Citation »

                                                                                                    Reviews neural systems and neurotransmitters involved in sleep and wakefulness, as well as mechanisms regulating transitions between states. Discusses effects of commonly used medications on sleep circuitry. Includes helpful illustrations.

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                                                                                                    • Hobson, J. A., and E. F. Pace-Schott. 2002. The cognitive neuroscience of sleep: Neuronal systems, consciousness and learning. Nature Reviews Neuroscience 3.9: 679–693.

                                                                                                      DOI: 10.1038/nrn915Save Citation »Export Citation »E-mail Citation »

                                                                                                      Reviews the neural basis of sleep with a focus on EEG oscillations, the relationship between sleep and consciousness, and the role of sleep in neural plasticity (the ability of groups of neurons to undergo structural changes and reorganization). Includes helpful figures.

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                                                                                                      • Krueger, J. M., D. M. Rector, S. Roy, H. P. A. Van Dongen, G. Belenky, and J. Panksepp. 2008. Sleep as a fundamental property of neuronal assemblies. Nature Reviews Neuroscience 9:910–919.

                                                                                                        DOI: 10.1038/nrn2521Save Citation »Export Citation »E-mail Citation »

                                                                                                        Reviews literature to make a case that sleep is regulated locally in the brain (“local sleep”) in a manner that depends upon how much a particular area was used during prior waking. Discusses a new mathematical model of local to global sleep transitions and theoretical perspectives on sleep functions. Includes useful illustrations.

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                                                                                                        • Monti, J. M. 2013. The neurotransmitters of sleep and wake, a physiological review series. Sleep Medicine Reviews 17:313–315.

                                                                                                          DOI: 10.1016/j.smrv.2013.02.004Save Citation »Export Citation »E-mail Citation »

                                                                                                          A brief overview of the neural circuitry, neurotransmitter systems, and circulating factors involved in sleep and wake states.

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                                                                                                          • Pace-Schott, E. F., and J. A. Hobson. 2002. The neurobiology of sleep: Genetic mechanisms, cellular neurophysiology and subcortical networks. Nature Reviews Neuroscience 3.8: 591–605.

                                                                                                            DOI: 10.1038/nrn895Save Citation »Export Citation »E-mail Citation »

                                                                                                            Reviews the neural basis of the sleep-wake cycle with a focus on genetics and neural circuitry. Includes helpful figures.

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                                                                                                            • Saper, C. B., P. M. Fuller, N. P. Pedersen, J. Lu, and T. E. Scammell. 2010. Sleep state switching. Neuron 68.6: 1023–1042.

                                                                                                              DOI: 10.1016/j.neuron.2010.11.032Save Citation »Export Citation »E-mail Citation »

                                                                                                              Comprehensive review of brain networks involved in wake and sleep promotion. Discusses a model of sleep state transitions, reviews evidence supporting the model, and discusses regulating factors. Includes an updated exposition on the biology of the “flip-flop” sleep switch and general applications of the reciprocal inhibition theory of behavioral state control (in which transitions between REM and NREM sleep result from dynamics between mutually inhibitory groups of neurons).

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                                                                                                              • Siegel, J. M. 2009. The neurobiology of sleep. Seminars in Neurology 29:277–296.

                                                                                                                DOI: 10.1055/s-0029-1237118Save Citation »Export Citation »E-mail Citation »

                                                                                                                Reviews the neuroanatomy and neurochemistry underlying NREM and REM sleep stages, changes in muscle tone during sleep, and narcolepsy. Includes many figures.

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                                                                                                                • Watson, C. J., H. A. Baghdoyan, and R. Lydic. 2012. Neuropharmacology of sleep and wakefulness: Update. Sleep Medicine Clinics 7:469–486.

                                                                                                                  DOI: 10.1016/j.jsmc.2012.06.010Save Citation »Export Citation »E-mail Citation »

                                                                                                                  Reviews the neurochemical basis of the sleep-wake cycle from a drug development perspective. Includes figures.

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                                                                                                                  Reviews on Specific Neuromodulators of Behavioral State Control

                                                                                                                  Since the discovery of specific monoamine (serotonin and norepinephrine)-producing groups of cells in the brainstem by Dahlstrom and Fuxe in 1964, the roles of many additional substances have been linked to behavioral state control. The importance of acetylcholine (e.g., Platt and Riedel 2011) and norepinephrine and serotonin (e.g., Monti 2010), originally postulated to control REM-NREM transitions in the reciprocal inhibition model (see Models and Theories of Sleep Neurobiology), continues to be corroborated. However, added to these are many additional neurochemicals including excitatory and inhibitory amino acids such as glutamate and GABA, purines such as adenosine (Porkka-Heiskanen and Kalinchuk 2011) and other monoamines such as histamine (e.g., Thakkar 2011) as well as large neuropeptides such as hormones and cytokines (e.g., Dyzma, et al. 2010). Among the most exciting recent findings has been the discovery that the neuropeptide orexin is a key stabilizer of the wake-promoting arousal systems (see Sakurai, et al. 2010).

                                                                                                                  • Berridge, C. W., B. E. Schmeichel, and R. A. España. 2012. Noradrenergic modulation of wakefulness/arousal. Sleep Medicine Reviews 16:187–197.

                                                                                                                    DOI: 10.1016/j.smrv.2011.12.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                    An in-depth review of the norepinephrine neurotransmitter system and its role in wakefulness, including mechanisms of its different receptors. Discusses the clinical relevance of this system. Includes figures.

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                                                                                                                    • Dyzma, M., K. Z. Boudjeltia, B. Faraut, and M. Kerkhofs. 2010. Neuropeptide Y and sleep. Sleep Medicine Reviews 14:161–165.

                                                                                                                      DOI: 10.1016/j.smrv.2009.09.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                      Briefly reviews the neurotransmitter molecule neuropeptide Y, including its chemical and metabolic characteristics, receptor profile, and effects on sleep.

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                                                                                                                      • Monti, J. M. 2010. Serotonin control of sleep-wake behavior. Sleep Medicine Reviews 15:269–281.

                                                                                                                        DOI: 10.1016/j.smrv.2010.11.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                        Introduces the general neuroanatomical and neurochemical bases of sleep and wake states, then provides an in-depth review of the serotonin neurotransmitter system and its role in regulation of wakefulness and REM sleep. Discusses the clinical relevance of the serotonin system.

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                                                                                                                        • Platt, B., and G. Riedel. 2011. The cholinergic system, EEG and sleep. Behavioral Brain Research 221:499–504.

                                                                                                                          DOI: 10.1016/j.bbr.2011.01.017Save Citation »Export Citation »E-mail Citation »

                                                                                                                          An overview of the role of the acetylcholine neurotransmitter system in sleep and wake states, with a focus on electroencephalographic (EEG) measures of sleep. Covers the link between cholinergic deficits and sleep disorders, epilepsy, and dementia.

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                                                                                                                          • Porkka-Heiskanen, T., and A. V. Kalinchuk. 2011. Adenosine, energy metabolism and sleep homeostasis. Sleep Medicine Review 15:123–135.

                                                                                                                            DOI: 10.1016/j.smrv.2010.06.005Save Citation »Export Citation »E-mail Citation »

                                                                                                                            Provides a general overview of sleep and its regulation, then focuses on the molecule adenosine as a leading candidate mechanism for the homeostatic regulation of sleep (Process S).

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                                                                                                                            • Sakurai, T., M. Mieda, and N. Tsujino. 2010. The orexin system: Roles in sleep/wake regulation. Annals of the New York Academy of Sciences 1200:149–161.

                                                                                                                              DOI: 10.1111/j.1749-6632.2010.05513.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                              An overview of the orexin neurotransmitter system, with an emphasis on its involvement in sleep and wakefulness. Describes the relationship between orexin deficiency and narcolepsy and discusses the therapeutic potential of orexin-based drugs. Includes figures.

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                                                                                                                              • Szymusiak, R., and D. McGinty. 2008. Hypothalamic regulation of sleep and arousal. Annals of the New York Academy of Sciences 1129:275–286.

                                                                                                                                DOI: 10.1196/annals.1417.027Save Citation »Export Citation »E-mail Citation »

                                                                                                                                Reviews literature regarding sleep-regulatory neurons in the preoptic area of the hypothalamus. Includes coverage of the interaction of these neurons with other sleep regulatory systems and their involvement in the homeostatic regulation of sleep/wake states. Includes figures.

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                                                                                                                                • Thakkar, M. M. 2011. Histamine in the regulation of wakefulness. Sleep Medicine Reviews 15:65–74.

                                                                                                                                  DOI: 10.1016/j.smrv.2010.06.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                  An in-depth review of the monoamine neurotransmitter histamine and its involvement in regulating sleep and wake states. Covers pharmacological, electrophysiological, molecular, and lesion/inactivation studies.

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                                                                                                                                  Reviews on the Genetics of Sleep

                                                                                                                                  Among the most exciting new approaches to sleep medicine have been genetic approaches, for which several general reviews are listed below (Bamne, et. al. 2010; Barclay and Gregory 2013; Kelly and Bianchi 2012). Microarray techniques have allowed screening of DNA for large numbers of genes at one time (see Cirelli 2009). Most recently, genome-wide association studies (GWAS) in humans (studies in which the entire genomes of individuals with and without a particular disease or condition are surveyed to find genes associated with that condition) have been employed to begin to seek out the genetic bases of sleep disorders such as narcolepsy and restless legs syndrome (see Raizen and Wu 2011). Specific single nucleotide polymorphisms (SNPs) involved in sleep-wake control, such as those for the adenosine A2A receptor (Landolt 2011), have been identified. SNPs are one or several mutations in one part of a specific gene that occurs in a large number of persons such that a groups with and without this mutation can be compared.

                                                                                                                                  • Bamne, M. N., H. Mansour, T. H. Monk, D. J. Buysse, and V. L. Nimagaonkar. 2010. Approaches to unravel the genetics of sleep. Sleep Medicine Reviews 14:397–404.

                                                                                                                                    DOI: 10.1016/j.smrv.2010.01.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                    Explains the current techniques used to study human genetics and reviews the findings pertaining to sleep of studies employing these methods.

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                                                                                                                                    • Barclay, N. L., and A. M. Gregory. 2013. Quantitative genetic research on sleep: A review of normal sleep, sleep disturbances and associated emotional, behavioural and health-related difficulties. Sleep Medicine Reviews 17:29–40.

                                                                                                                                      DOI: 10.1016/j.smrv.2012.01.008Save Citation »Export Citation »E-mail Citation »

                                                                                                                                      Reviews data from twin studies investigating genetic and environmental contributions to normal sleep characteristics, sleep problems and disorders, and associations between sleep disturbances and emotional, behavioral, and health-related problems in children and adults.

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                                                                                                                                      • Cirelli, C. 2009. The genetic and molecular regulation of sleep: From fruit flies to humans. Nature Reviews Neuroscience 10.8: 549–560.

                                                                                                                                        DOI: 10.1038/nrn2683Save Citation »Export Citation »E-mail Citation »

                                                                                                                                        Reviews literature regarding the genetics of sleep in humans and model organisms. Includes genes involved in circadian regulation, neurotransmission, and various sleep disorders. Additionally reviews wake and sleep-related gene transcription in the brain. Includes helpful figures and tables.

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                                                                                                                                        • Kelly, J. M., and M. T. Bianchi. 2012. Mammalian sleep genetics. Neurogenetics 13.4: 287–326.

                                                                                                                                          DOI: 10.1007/s10048–012–0341-xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                          An extensive review of sleep-related phenotypes of over fifty transgenic animal models, with an emphasis on transgenic mice. Includes effects of genetic manipulations on sleep architecture, circadian regulation, and response to sleep deprivation. Comments on methodology and data interpretation. Includes useful tables.

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                                                                                                                                          • Landolt, H. 2011. Genetic determination of sleep EEG profiles in healthy humans. Progress in Brain Research 193:51–61.

                                                                                                                                            DOI: 10.1016/B978-0-444-53839-0.00004-1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                            Reviews the trait-like nature and heritability of waking and sleep EEG, as well as genetic polymorphisms affecting sleep and EEG in humans. Includes some figures and tables.

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                                                                                                                                            • Raizen, D. M. I., and M. N. Wu. 2011. Genome-wide association studies of sleep disorders. Chest 139:446–452.

                                                                                                                                              DOI: 10.1378/chest.10-1313Save Citation »Export Citation »E-mail Citation »

                                                                                                                                              Reviews genome-wide association studies of narcolepsy and restless legs syndrome.

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                                                                                                                                              • Sehgal, A., and E. Mignot. 2011. Genetics of Sleep and Sleep Disorders. Cell 146:194–207.

                                                                                                                                                DOI: 10.1016/j.cell.2011.07.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                Reviews the molecular and genetic mechanisms involved in sleep as assessed in model organisms, as well as the genetic basis of human sleep disorders. Discusses the overlap between sleep-related genes in model organisms and humans. Includes some figures and tables.

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                                                                                                                                                New Discoveries on the Neurobiology of Sleep

                                                                                                                                                Many new findings in the past two decades have broadened our concepts of sleep and wakefulness. For example, emerging evidence that adenosine is an endogenous somnogen (sleep-promoting factor) involved in sleep homeostasis or Process S (Porkka-Heiskanen, et al. 1997) has provided a possible mechanism for the notion of “local sleep” (see Halassa, et al. 2009). To the cholinergic and aminergic cell groups originally posited to control the REM/NREM cycle have been added complex networks of inhibitory neurons (neurons that inhibit other neurons) and dis-inhibitory neurons (those that release other neurons from inhibition by inhibiting the neurons responsible for their inhibition), both of which release GABA (Boissard, et al. 2002; Lu, et al. 2006). Discoveries at the molecular level have led to novel theories that functions of sleep may include providing the optimal physiological conditions for efficient intracellular detection and disposition (Naidoo, et al. 2005) as well as mechanical clearance (Xie, et al. 2013) of misfolded proteins. These findings in turn suggest mechanisms whereby poor sleep might contribute to the pathophysiology of neurodegenerative diseases.

                                                                                                                                                • Boissard, R., D. Gervasoni, M. H. Schmidt, B. Barbagli, P. Fort, and P. H. Luppi. 2002. The rat ponto-medullary network responsible for paradoxical sleep onset and maintenance: A combined microinjection and functional neuroanatomical study. European Journal of Neuroscience 16:1959–1973.

                                                                                                                                                  DOI: 10.1046/j.1460-9568.2002.02257.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                  Elucidation of brainstem GABA pathways for the control of the REM-NREM alternation.

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                                                                                                                                                  • Halassa, M. M., C. Florian, T. Fellin, et al. 2009. Astrocytic modulation of sleep homeostasis and cognitive consequences of sleep loss. Neuron 61:213–219.

                                                                                                                                                    DOI: 10.1016/j.neuron.2008.11.024Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                    Description of the mechanism by which astrocytes (non-neuronal brain cells that support neurons) play a key role in the local accumulation of adenosine leading to increased homeostatic sleep pressure (increased Process S).

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                                                                                                                                                    • Lu, J., D. Sherman, M. Devor, and C. B. Saper. 2006. A putative flip-flop switch for control of REM sleep. Nature 441.7093: 589–594.

                                                                                                                                                      DOI: 10.1038/nature04767Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                      Description, in the rat, of inhibitory neuronal populations in the mesopontine brainstem (region including parts of both pons and midbrain) that could control the alternation of REM and NREM states.

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                                                                                                                                                      • Naidoo, N., W. Giang, R. J. Galante, and A. I. Pack. 2005. Sleep deprivation induces the unfolded protein response in mouse cerebral cortex. Journal of Neurochemistry 92:1150–1157.

                                                                                                                                                        DOI: 10.1111/j.1471-4159.2004.02952.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                        Demonstration that prolonged wakefulness induces a cellular stress response in the endoplasmic reticulum known to slow down protein synthesis.

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                                                                                                                                                        • Porkka-Heiskanen, T., R. E. Strecker, M. Thakkar, A. A. Bjorkum, R. W. Greene, and R. W. McCarley. 1997. Adenosine: A mediator of the sleep-inducing effects of prolonged wakefulness. Science 276:1265–1268.

                                                                                                                                                          DOI: 10.1126/science.276.5316.1265Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                          First description of evidence that extracellular adenosine may act as an endogenous somnogen (sleep-promoting factor that builds up with continued wakefulness) controlling the sleep homeostatic (Process S) arm of the two-process model of sleep (see Models and Theories of Sleep Neurobiology).

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                                                                                                                                                          • Sherin, J. E., P. J. Shiromani, R. W. Mccarley, and C. B. Saper. 1996. Activation of ventrolateral preoptic neurons during sleep. Science 271:216–219.

                                                                                                                                                            DOI: 10.1126/science.271.5246.216Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                            Discovery of a sleep-promoting region in the ventrolateral preoptic nucleus of the hypothalamus that led to the “sleep-switch” (a.k.a. flip-flop or reciprocal inhibition) model which posits that inhibitory interactions between wake-promoting and sleep-promoting neurons control transitions between sleep and wake states.

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                                                                                                                                                            • Xie, L., H. Kang, Q. Xu, M. J. Chen, and Y. Liao, et al. 2013. Sleep drives metabolite clearance from the adult brain. Science 342:373–377.

                                                                                                                                                              DOI: 10.1126/science.1241224Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                              Finding of increased interstitial fluid (fluid in spaces between brain cells) and associated increase in metabolite (cellular waste) clearance rate during sleep. Provides a putative mechanism whereby sleep disruption could lead to dementias that, like Alzheimer’s disease, are associated with abnormal accumulation of substances produced by the neurons themselves.

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                                                                                                                                                              PET and fMRI Studies of Sleep

                                                                                                                                                              A group of pioneering positron-emission tomography (PET) studies in the late 1990s provided convergent evidence that, following widespread diminished activation at the transition from waking to NREM sleep, the anterior midline limbic-system structures of the brain reactivate to near-waking levels during REM (Braun, et al. 1997; Braun, et al. 1998; Maquet, et al. 1996; Nofzinger, et al. 1997). These findings provided impetus for new concepts on the emotional processing functions of sleep as well as putative neural bases of dream phenomenology. These findings have since been corroborated and extended by fMRI studies (e.g., Kaufmann, et al. 2006; Wehrle, et al. 2007).

                                                                                                                                                              • Braun, A. R., T. J. Balkin, N. J. Wesensten, et al. 1997. Regional cerebral blood flow throughout the sleep-wake cycle. Brain 120:1173–1197.

                                                                                                                                                                DOI: 10.1093/brain/120.7.1173Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                Characterization of brain activity across the sleep-wake cycle using PET.

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                                                                                                                                                                • Braun, A. R., T. J. Balkin, N. J. Wesensten, et al. 1998. Dissociated pattern of activity in visual cortices and their projections during human rapid eye-movement sleep. Science 279:91–95.

                                                                                                                                                                  DOI: 10.1126/science.279.5347.91Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                  Demonstration of a functional disconnection between areas of the visual processing pathway during REM sleep using PET. Findings have implications regarding dream-related visual imagery.

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                                                                                                                                                                  • Kaufmann, C., R. Wehrle, T. C. Wetter, F. Holsboer, D. P. Auer, T. Pollmacher, and M. Czisch. 2006. Brain activation and hypothalamic functional connectivity during human non-rapid eye movement sleep: An EEG/fMRI study. Brain 129:655–667.

                                                                                                                                                                    DOI: 10.1093/brain/awh686Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                    Characterization of brain activity during deepening NREM sleep using fMRI.

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                                                                                                                                                                    • Maquet, P., C. Degueldre, G. Delfiore, et al. 1997. Functional neuroanatomy of human slow wave sleep. Journal of Neuroscience 17:2807–2812.

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                                                                                                                                                                      Characterization of brain activity during slow wave sleep (SWS) using PET.

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                                                                                                                                                                      • Maquet, P., J. M. Peters, J. Aerts, et al. 1996. Functional neuroanatomy of human rapid-eye-movement sleep and dreaming. Nature 383:163–166.

                                                                                                                                                                        DOI: 10.1038/383163a0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                        Characterization of brain activity during REM sleep using PET.

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                                                                                                                                                                        • Nofzinger, E. A., M. A. Mintun, M. B. Wiseman, D. J. Kupfer, and R. Y. Moore. 1997. Forebrain activation in REM sleep: An FDG PET study. Brain Research 770:192–201.

                                                                                                                                                                          DOI: 10.1016/S0006-8993(97)00807-XSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                          Demonstration, using PET, that limbic (more primitive emotion and memory related) and paralimbic (in proximity to limbic) forebrain areas are relatively more active during REM sleep than wake.

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                                                                                                                                                                          • Wehrle, R., C. Kaufmann, T. C. Wetter, et al. 2007. Functional microstates within human REM sleep: First evidence from fMRI of a thalamocortical network specific for phasic REM periods. European Journal of Neuroscience 25.3: 863–871.

                                                                                                                                                                            DOI: 10.1111/j.1460-9568.2007.05314.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                            Demonstration that tonic (times without rapid eye movements) and phasic (times with rapid eye movements) REM sleep differ with respect to processing external information and the identification of a specific network associated with phasic REM sleep.

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                                                                                                                                                                            Circadian Modulation of Sleep and Waking

                                                                                                                                                                            Like many bodily processes, the sleep-wake cycle follows a circadian (near twenty-four-hour) rhythm. Following the discovery that the suprachiasmatic nucleus (SCN) of the anterior hypothalamus provides a master clock for mammalian circadian rhythms using experiments that first removed and then replaced the SCN, much has been discovered regarding the intricate mechanisms by which this clock operates. Among such findings have been the elucidation of the processes inside the cell nucleus and cytoplasm whereby genes are transcribed from DNA to messenger RNA (transcription) and messenger RNA is “read” to produce proteins (translation) allowing a single cell to provide a reliable twenty-four-hour signal. Also discovered was the surprising fact that such molecular clocks also occur elsewhere in the brain as well as in many peripheral (outside the central nervous system) tissues, and the discovery of the mechanism by which light is able to entrain (synchronize with the environment) the master circadian clock.

                                                                                                                                                                            Reviews on Mammalian Circadian Regulation

                                                                                                                                                                            The topic of mammalian circadian biology is vast and a student beginning to explore this field is well advised to thoroughly digest several of the excellent reviews available on different major subsets of this topic. These can be divided into descriptions of the molecular mechanism of cellular circadian output, the neural networks by which this signal is communicated within the central nervous system (brain and spinal cord) as well as to the periphery, the entrainment of circadian rhythms by light, and the neural roles of melatonin (the “darkness hormone” secreted at night from the pineal gland).

                                                                                                                                                                            Molecular Mechanisms of Cellular Circadian Output

                                                                                                                                                                            One of the most exciting recent advances in both neuroscience and molecular biology has been the discovery of the transcriptional and translational negative-feedback (self-inhibiting) system that allows a single cell to output a reliable signal with a twenty-four-hour period. This molecular mechanism is reviewed in Fisher, et al. 2013; Landgraf, et al. 2012; Piggins and Guilding 2011. At the behavioral level, these mechanisms are reflected in both normal sleep and circadian sleep disorders (e.g., Von Schantz 2008)

                                                                                                                                                                            • Fisher, S. P., R. G. Foster, and S. N. Peirson. 2013. The circadian control of sleep. Handbook of Experimental Pharmacology 217:157–183.

                                                                                                                                                                              DOI: 10.1007/978-3-642-25950-0_7Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                              An accessible review of sleep regulation, with a focus on the contribution of the circadian system. Includes the role of specific clock genes (the specific genes involved in the molecular circadian clock), light, melatonin, and social cues. Includes some figures.

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                                                                                                                                                                              • Landgraf, D., A. Shostak, and H. Oster. 2012. Clock genes and sleep. European Journal of Physiology 463:3–14.

                                                                                                                                                                                DOI: 10.1007/s00424-011-1003-9Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                Reviews the interaction between the circadian (Process C) and homeostatic (Process S) components of sleep regulation, with a particular focus on the role of clock genes in these processes.

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                                                                                                                                                                                • Piggins H. D., and C. Guilding. 2011. The neural circadian system of mammals. Essays in Biochemistry 49:1–17.

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                                                                                                                                                                                  Reviews the transcriptional and translational molecular mechanisms producing interlocking positive (self-exciting) and negative (self-inhibiting) feedback systems controlling the mammalian circadian rhythm.

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                                                                                                                                                                                  • Von Schantz, M. 2008. Phenotypic effects of genetic variability in human clock genes on circadian and sleep parameters. Journal of Genetics 87:513–519.

                                                                                                                                                                                    DOI: 10.1111/j.1460-9568.2009.06723.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                    Reviews animal and human literature regarding the effect of variations in clock genes on diurnal preferences (whether an animal is active at night or day), circadian-rhythm sleep disorders, and sleep homeostasis. Discusses the conservation of clock genes across evolutionary history.

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                                                                                                                                                                                    Networks Transmitting Circadian Signal within the Central Nervous System (CNS) and Periphery

                                                                                                                                                                                    Along with advances in understanding the molecular genetic bases of circadian rhythms have come new discoveries on how the synchronous (all at the same time) circadian output from the relatively small number of cells in the SCN is transformed into the many different circadian rhythms of behavior and physiology, as reviewed in Saper, et al. 2005a. Key among these findings have been the identification of divergent pathways downstream from the SCN in the hypothalamus and the discovery of molecular oscillators (“clocks” that regularly output a signal after every certain period of time) in many different tissues of the CNS and periphery.

                                                                                                                                                                                    • Dibner, C., U. Schibler, and U. Albrecht. 2010. The mammalian circadian timing system: Organization and coordination of central and peripheral clocks. Annual Review of Physiology 72:517–549.

                                                                                                                                                                                      DOI: 10.1146/annurev-physiol-021909-135821Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                      A comprehensive review of both central and peripheral components of the circadian oscillatory system. Covers the mechanisms by which the master clock in the SCN synchronizes oscillators throughout the body. Discusses interactions between circadian rhythms and the reward system in the brain. Includes helpful illustrations.

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                                                                                                                                                                                      • Dijk, D. J., and S. N. Archer. 2010. PERIOD3, circadian phenotypes and sleep homeostasis. Sleep Medicine Reviews 14:151–160.

                                                                                                                                                                                        DOI: 10.1016/j.smrv.2009.07.002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                        Reviews the effects of variation in the circadian clock gene PER3 on several parameters including sleep timing, EEG characteristics, and cognitive performance. Discusses a conceptual model of circadian and homeostatic regulation of sleep capable of explaining these findings. Includes useful figures.

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                                                                                                                                                                                        • Franken, P., and D. J. Dijk. 2009. Circadian clock genes and sleep homeostasis. European Journal of Neuroscience 29:1820–1829.

                                                                                                                                                                                          DOI: 10.1111/j.1460-9568.2009.06723.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                          Reviews evidence from human and animal studies for a role of clock genes in homeostatic regulation of sleep. Discusses the shared molecular pathways of circadian and homeostatic processes. Includes helpful figures.

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                                                                                                                                                                                          • Gillette, M. U., and S. M. Abbot. 2009. Biological timekeeping. Journal of Clinical Sleep Medicine 4.2: 99–110.

                                                                                                                                                                                            DOI: 10.1016/j.jsmc.2009.01.005Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                            Reviews the neuroanatomical and genetic basis of the central circadian rhythm system, as well as regulatory molecules acting on this system at various times of day.

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                                                                                                                                                                                            • Rosenwasser, A. M. 2009. Functional neuroanatomy of sleep and circadian rhythms. Brain Research Reviews 61:281–306.

                                                                                                                                                                                              DOI: 10.1016/j.brainresrev.2009.08.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                              Reviews the neurobiological bases of sleep-wake states and circadian rhythms, with an emphasis on the reciprocal (two-way) interactions between these systems and the implications of this interrelationship.

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                                                                                                                                                                                              • Saper, C. B. 2013. The central circadian timing system. Current Opinion in Neurobiology 23:747–751.

                                                                                                                                                                                                DOI: 10.1016/j.conb.2013.04.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                Together with the above two articles, reviews neuroanatomic pathways within the hypothalamus leading from the master clock in the SCN to specific nuclei (groups of neurons) controlling circadian rhythms of sleep, feeding, and other behaviors.

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                                                                                                                                                                                                • Saper, C. B., J. Lu, T. C. Chou, and J. Gooley. 2005a. The hypothalamic integrator for circadian rhythms. Trends in Neurosciences 28.3: 152–157.

                                                                                                                                                                                                  DOI: 10.1016/j.tins.2004.12.009Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                  Reviews the neural mechanisms by which SCN’s daily signals are converted into behavioral and physiological rhythms including the sleep-wake cycle. Includes helpful figures.

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                                                                                                                                                                                                  • Saper, C. B., T. E. Scammell, and J. Lu. 2005b. Hypothalamic regulation of sleep and circadian rhythms. Nature 437.7063: 1257–1263.

                                                                                                                                                                                                    DOI: 10.1038/nature04284Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                    Reviews the neural circuitry underlying the generation and regulation of the sleep-wake cycle. Includes helpful figures.

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                                                                                                                                                                                                    Entrainment of Circadian Rhythms by Light

                                                                                                                                                                                                    Much has been recently learned about the mechanism by which ambient light can entrain (reset to synchrony with the environment) the SCN oscillator. The key discovery was of the nonvisual photopigments (chemicals that react to light but aren’t involved with vision per se) such as melanopsin in retinal ganglion cells (specific cells in the retina), along with their uniquely preferred wavelengths (reviewed in Hughes, et al. 2012). With this discovery has come renewed interest in the other behavioral and physiological effects of light in addition to its entrainment effects (reviewed in Duffy and Czeisler 2009), such as its effects on mood and cognition (Cajochen 2007).

                                                                                                                                                                                                    Melatonin

                                                                                                                                                                                                    Measurement of the pineal hormone melatonin has proven to have immense value in the study of circadian rhythms. Its steep increase in the blood as sleep approaches has been shown to be the most reliable anchor point for experimental measurement of normal circadian oscillations. It is also important in diagnosing clinical disorders such as advanced (moved earlier) or delayed (moved later) sleep phase syndrome as well as the treatment of these disorders using phototherapy (using light to reset the circadian clock). In addition, feedback effects of melatonin on its receptors in the SCN and other central nervous system sites are important to its use as an exogenous (externally applied) regulator of circadian-based sleep problems such as jet lag. We cite one recent and important review of melatonin effects on circadian rhythms (Zawilska, et al. 2009) as well as another that discusses one of its other important biological functions as a sensor of “photoperiod” or day length (Trivedi and Kumar 2014).

                                                                                                                                                                                                    • Trivedi, A. K., and V. Kumar. 2014. Melatonin: An internal signal for daily and seasonal timing. Indian Journal of Experimental Biology 52.5: 425–437.

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                                                                                                                                                                                                      The fact that melatonin secretion rhythms in the pineal gland can transmit information on photoperiod to the brain makes it an important coordinator of seasonal rhythms such as reproduction and hibernation. This review touches both on its role in circadian physiology as well as seasonal physiology.

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                                                                                                                                                                                                      • Zawilska, J. B., D. J. Skene, and J. Arendt. 2009. Physiology and pharmacology of melatonin in relation to biological rhythms. Pharmacological Reports 61:383–410.

                                                                                                                                                                                                        DOI: 10.1016/S1734-1140(09)70081-7Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                        An in-depth review of melatonin, including its metabolic properties, receptors, and mechanisms underlying its regulation. Additionally covers the physiological functions of this hormone, including its role in sleep and circadian rhythms. Discusses the clinical relevance of the melatonin system.

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                                                                                                                                                                                                        Original Reports on Circadian Rhythms

                                                                                                                                                                                                        The above reviews are invaluable reading to grasp the wide scope of circadian science. However, original reports provide fascinating insight into how these component discoveries arose. For example, the undisputed role of the SCN in circadian control was established by transplantation studies in the hamster (Ralph, et al. 1990). The pathways by which the circadian clock in the SCN controls sleep-wake cycles by way of the arousal system (e.g., the noradrenergic locus coeruleus) is reported in Aston-Jones, et al. 2001. Whereas early investigation of the circadian clock in humans, which relied on isolating subjects from external stimuli, suggested that the endogenous period exceeded twenty-four hours, new techniques such as forced desynchrony that do not require such total isolation revealed just how closely our internal clock matches the astronomical day (Czeisler, et al. 1999). The nonvisual circadian photopigment melanopsin was described in the early 2000s by several groups (e.g., Berson, et al. 2002; Gooley, et al. 2001; Hannibal, et al. 2002; Hattar, et al. 2002), thereby providing an understanding of how the eye can be the peripheral sense organ for both the visual and the circadian systems.

                                                                                                                                                                                                        • Aston-Jones, G., S. Chen, Y. Zhu, and M. L. Oshinsky. 2001. A neural circuit for circadian regulation of arousal. Nature Neuroscience 4:732–738.

                                                                                                                                                                                                          DOI: 10.1038/89522Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                          Discovery of a noradrenergic link between the SCN, which is the central pacemaker of the circadian system, and arousal centers in the brainstem.

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                                                                                                                                                                                                          • Berson, D. M., F. A. Dunn, and M. Takao. 2002. Phototransduction by retinal ganglion cells that set the circadian clock. Science 295:1070–1073.

                                                                                                                                                                                                            DOI: 10.1126/science.1067262Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                            Demonstration that retinal ganglion cells innervating the SCN are sensitive to light, suggesting that these cells are the photoreceptors of the circadian rhythm system.

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                                                                                                                                                                                                            • Czeisler, C. A., J. F. Duffy, T. L. Shanahan, et al. 1999. Stability, precision, and near 24 hour period of the human circadian pacemaker. Science 284:2177–2181.

                                                                                                                                                                                                              DOI: 10.1126/science.284.5423.2177Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                              Use of a forced desynchrony protocol, by which the sleep-wake cycle is uncoupled from other biological rhythms, to show that the human circadian period was closer to twenty-four hours than previously thought. Forced desynchrony involves placing research subjects in twenty- or twenty-eight-hour “days” which exceed the ability of the master clock in the SCN to adjust (entrain).

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                                                                                                                                                                                                              • Gooley, J. J., J. Lu, T. C. Chou, T. E. Scamell, and C. B. Saper. 2001. Melanopsin in cells of origin of the retinohypothalamic tract. Nature Neuroscience 4:1165.

                                                                                                                                                                                                                DOI: 10.1038/nn768Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                Demonstration that the majority of retinal ganglion cells projecting to the SCN contain melanopsin, suggesting that this chemical is the photopigment of the circadian rhythm system.

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                                                                                                                                                                                                                • Hannibal, J., P. Hindersson, S. M. Knudsen, B. Georg, and J. Fahrenkrug. 2002. The photopigment melanopsin is exclusively present in pituitary adenylate cyclase-activating polypeptide-containing retinal ganglion cells of the retinohypothalamic tract. The Journal of Neuroscience 22:RC191 (1–7).

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                                                                                                                                                                                                                  Demonstration that melanopsin is located exclusively in retinal cells projecting to the SCN, again suggesting that melanopsin is the photoreceptive pigment for the entrainment of the circadian rhythms in mammals.

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                                                                                                                                                                                                                  • Hattar, S., H. W. Liao, M. Takao, D. M. Berson, and K. W. Yau. 2002. Melanopsin-containing retinal ganglion cells: Architecture, projections, and intrinsic photosensitivity. Science 295:1065–1070.

                                                                                                                                                                                                                    DOI: 10.1126/science.1069609Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                    Along with the above four reports, was among the first to show that melanopsin is the nonvisual circadian photochemical.

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                                                                                                                                                                                                                    • Ralph, M. R., R. G. Foster, and F. C. Davis. 1990. Transplanted suprachiasmatic nucleus determines circadian period. Science 27:975–978.

                                                                                                                                                                                                                      DOI: 10.1126/science.2305266Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                      Demonstration that SCN controls circadian rhythm.

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                                                                                                                                                                                                                      Sleep and Neuronal Oscillations in the Brain

                                                                                                                                                                                                                      REM and NREM sleep stages alternate in a cyclical manner with a period of about ninety minutes in humans and a shorter duration in the cat and rat. This regularity is indicative of a reliable underlying neuronal oscillator (any structure that produces a signal with a fixed period). Such oscillation is one of a number of alternating patterns produced by the central nervous system that organize the physiology of sleep and waking. The period (time between signals) of such biological rhythms extend upward to twenty-four hours (i.e., “circadian”) and longer (e.g., seasonal rhythms) and also extend downward through the REM-NREM “ultradian” (more than once per day) cycle to the neuronal oscillations measured using electroencephalography (EEG) that range from one cycle per several seconds (<1 Hz) to over 80 Hz. Very different types of molecular, cellular, neural circuit, and hormonal mechanisms control cycles of different periods. Nonetheless, the phenomenon of cyclicity (behavior or physiological events happening every certain period of time) is one of the key characteristics of neuronal, sleep-wake and, indeed, all biological behavior.

                                                                                                                                                                                                                      Reviews on the Neuronal Oscillators in the Mammalian Brain

                                                                                                                                                                                                                      Because the EEG was among the first devices used to study and characterize the physiology of sleep, researchers have long focused on the electromagnetic oscillatory rhythms of the brain and their potential biological significance. In neuronal networks an “oscillator” or “oscillation” refers to events happening every certain period of time often in the range of 0.1 Hz (once every ten seconds) to 80 Hz (eighty times per second). In the case of sleep rhythms, a key discovery by investigators such as Mircea Steriade (Steriade 2000, Steriade 2006) was that thalamo-cortical oscillations (repeating firing patterns among loops of interconnected neurons in the thalamus and cortex) underlie the regular rhythms of NREM sleep (Timofeev and Chauvette 2011). Also elucidated were membrane mechanisms that control these rhythms at the cellular level (Crunelli, et al. 2006). Much recent interest in the cognitive functions of sleep has focused on these rhythms. These include the memory consolidation effects attributed to delta (0.5–4.0 Hz) (Crunelli and Hughes 2010) and slow oscillations (<1 Hz) during slow wave sleep (SWS) as well as the possible role of faster NREM waveforms such as the spindle (a 12–15 Hz rhythm that first increases then decreases in amplitude and lasts at least 0.5 seconds) in transferring newly acquired information from the hippocampus to the cortex for storage (Dang-Vu 2012; De Gennaro and Ferrera 2003). More rapid gamma-frequency (30–80 Hz) oscillations (e.g., Buzsaki and Lopes da Silva 2012; Buzsaki and Wang 2012), typical of focused attention and cognition in waking, have been detected in REM sleep and attributed to the cognitive processes occurring during dreaming. Very recently, the ultra-slow oscillations (0.01–0.1 Hz) in the blood-oxygen level dependent (BOLD) signal of the fMRI have been used to describe differences in intrinsic connectivity (regions that activate and deactivate together) between different regions of the brain during sleep versus wakefulness. Recent studies suggest that certain oscillatory patterns may constitute intrinsic properties of neuronal tissue, and such findings have invigorated studies of local sleep in different portions of the brain or even in brain slices and artificial neuronal assemblies (Crunelli, et al. 2006, Pignatelli, et al. 2012).

                                                                                                                                                                                                                      • Buzsaki, G., and F. Lopes da Silva. 2012. High frequency oscillations in the intact brain. Progress in Neurobiology 98:241–249.

                                                                                                                                                                                                                        DOI: 10.1016/j.pneurobio.2012.02.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                        Reviews the literature regarding sleep-related high-frequency oscillations, with an emphasis on hippocampal sharp wave-ripple (150–250 Hz) events. Discusses relevance of these oscillations for memory consolidation and epilepsy.

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                                                                                                                                                                                                                        • Buzsaki, G., and X. J. Wang. 2012. Mechanisms of gamma oscillations. Annual Review of Neuroscience 35:203–225.

                                                                                                                                                                                                                          DOI: 10.1146/annurev-neuro-062111-150444Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                          Reviews literature regarding gamma (30–80 Hz) oscillations, including network models of this rhythm, cellular network mechanisms, and synchrony across brain areas. Includes helpful figures.

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                                                                                                                                                                                                                          • Crunelli, V., D. W. Cope, and S. W. Hughes. 2006. Thalamic T-type Ca2+ channels and NREM sleep. Cell Calcium 40:175–190.

                                                                                                                                                                                                                            DOI: 10.1016/j.ceca.2006.04.022Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                            Profiles the involvement of T-type calcium channels (a specific type of neuronal membrane channel that opens only under certain conditions) in the generation of alpha (8–13 Hz), theta (4–7 Hz), delta (0.5–4.0 Hz), spindle (12–15 Hz), and slow oscillation (<1 Hz) rhythms. Includes helpful figures.

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                                                                                                                                                                                                                            • Crunelli, V., and S. W. Hughes. 2010. The slow (<1 Hz) rhythm of non-REM sleep: A dialogue between three cardinal oscillators. Nature Neuroscience 13:9–17.

                                                                                                                                                                                                                              DOI: 10.1038/nn.2445Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                              Reviews the literature regarding characteristics and mechanisms of the sleep slow (<1 Hz) oscillation. Proposes that this oscillation is the product of interactions between an oscillator in the cortex and two oscillators in the thalamus. Includes useful figures.

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                                                                                                                                                                                                                              • Dang-Vu, T. T. 2012. Neuronal oscillations in sleep: Insights from functional neuroimaging. NeuroMolecular Medicine 14:154–167.

                                                                                                                                                                                                                                DOI: 10.1007/s12017-012-8166-1Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                Reviews brain activity associated with sleep spindles, slow waves, and REM sleep phasic activity as measured by functional neuroimaging. Additionally reviews the relationship between these oscillations and sensory information processing during sleep. Includes figures.

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                                                                                                                                                                                                                                • De Gennaro, L., and M. Ferrera. 2003. Sleep spindles: An overview. Sleep Medicine Reviews 7.5: 423–440.

                                                                                                                                                                                                                                  DOI: 10.1016/S1087-0792(02)00116-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                  A broad review of various sleep spindle characteristics, including temporal dynamics within and across sleep episodes, underlying mechanisms, role in information processing, changes across the lifespan, cortical topography (how they are distributed across the cortex), and their trait-like nature.

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                                                                                                                                                                                                                                  • Pignatelli, M., A. Beyeler, and X. Leinekugel. 2012. Neural circuits underlying the generation of theta oscillations. Journal of Physiology—Paris 106:81–92.

                                                                                                                                                                                                                                    DOI: 10.1016/j.jphysparis.2011.09.007Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                    Reviews mechanisms underlying the hippocampal theta (4–7 Hz) rhythm, including the neuronal circuitry involved and control of its generation by brainstem and hypothalamic areas. Includes figures.

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                                                                                                                                                                                                                                    • Steriade, M. 2000. Corticothalamic resonance, states of vigilance and mentation. Neuroscience 101:243–276.

                                                                                                                                                                                                                                      DOI: 10.1016/S0306-4522(00)00353-5Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                      Reviews types of oscillations in thalamo-cortical networks, their underlying neuronal substrates, and their relationships with behavioral states. Includes many figures.

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                                                                                                                                                                                                                                      • Steriade, M. 2006. Grouping of brain rhythms in corticothalamic systems. Neuroscience 137:1087–1106.

                                                                                                                                                                                                                                        DOI: 10.1016/j.neuroscience.2005.10.029Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                        Along with Steriade 2000, this article describes sleep spindles, delta (0.5–4.0 Hz) waves, and the slow (<1 Hz) oscillation. Discusses the effects of low frequency oscillations on brain plasticity and the role of the slow oscillation in grouping the other, faster, rhythms. Additionally reviews beta (14–29 Hz) and gamma (30–80 Hz) oscillations occurring during wake and REM sleep. Includes many figures.

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                                                                                                                                                                                                                                        • Timofeev, I., and S. Chauvette. 2011. Thalamocortical oscillations: Local control of EEG slow waves. Current Topics in Medicinal Chemistry 11:2457–2471.

                                                                                                                                                                                                                                          DOI: 10.2174/156802611797470376Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                          Reviews the circuitry and organization of the thalamo-cortical system. Briefly describes the various oscillations observed during NREM sleep and their underlying generating mechanisms, and then looks more in depth at properties of the slow oscillation including its origin, propagation, and underlying cellular mechanisms. Includes helpful figures.

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                                                                                                                                                                                                                                          Original Findings Regarding Neuronal Oscillations

                                                                                                                                                                                                                                          As in the case of circadian biology, it is most instructive to return to the original reports describing the discovery of specific oscillatory rhythms first in the brains of animals and then in the human. For example, discovery of the slow (<1 Hz) EEG oscillation arose from exacting techniques for intracellular recording in the brains of living cats (Steriade, et al. 1993). Once discovered, however, it was soon recognized in the human sleep EEG (Achermann and Borbély 1997). Similarly, following the discovery of gamma oscillations (30–80 Hz) in the animal brain, these were also detected in the human sleep EEG (Gross and Gotman 1999).

                                                                                                                                                                                                                                          • Achermann, P., and A. A. Borbély. 1997. Low frequency (<1 Hz) oscillations in the human sleep electroencephalogram. Neuroscience 81:213–222.

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                                                                                                                                                                                                                                            First documentation of the slow (<1 Hz) oscillation in the human EEG.

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                                                                                                                                                                                                                                            • Gross, D. W., and J. Gotman. 1999. Correlation of high-frequency oscillations with the sleep-wake cycle and cognitive activity in humans. Neuroscience 94:1005–1018.

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                                                                                                                                                                                                                                              One of the first demonstrations of the gamma (30–80 Hz) oscillation in the human brain during sleep.

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                                                                                                                                                                                                                                              • Steriade, M., A. Nunez, and F. Amzica. 1993. A novel slow (<1 Hz) oscillation of neocortical neurons in vivo: Depolarizing and hyperpolarizing components. Journal of Neuroscience 13.8: 3252–3265.

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                                                                                                                                                                                                                                                First identification of the slow oscillation in the cat.

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                                                                                                                                                                                                                                                Sleep and Endocrine Activity

                                                                                                                                                                                                                                                Sleep and the endocrine system are intimately linked via both the circadian (Morris, et al. 2012) and stress systems (Chrousos 2007). For example, growth hormone is preferentially secreted during SWS, melatonin secretion begins as sleep onset nears and cortisol (the hormone produced by the hypothalamic-pituitary-adrenal or HPA axis) peaks following awakening (Frenette, et al. 2012; Pannain and van Cauter 2007; Steiger 2003). Insufficient sleep has been shown to produce alterations in these same systems, leading to suppressed immunity (Gómez-González, et al. 2012), metabolic disorders including obesity and Type-II diabetes (Morris, et al. 2012), and cardiovascular disease (Al Dabal and Ba Hammam 2011).

                                                                                                                                                                                                                                                • Al Dabal, L., and A. S. Ba Hammam. 2011. Metabolic, endocrine, and immune consequences of sleep deprivation. The Open Respiratory Medicine Journal 5:31–43.

                                                                                                                                                                                                                                                  DOI: 10.2174/1874306401105010031Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                  Reviews literature regarding the effects of sleep deprivation on immune function, endocrine activity, and metabolism. Discusses the link between sleep deprivation and obesity in children and adults, as well as the link between sleep deprivation and diabetes, hypertension, and dyslipidemia (high levels of “bad cholesterol”).

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                                                                                                                                                                                                                                                  • Chrousos, G. P. 2007. Organization and integration of the endocrine system. Sleep Medicine Clinics 2.2: 125–145.

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                                                                                                                                                                                                                                                    Reviews the neuroendocrine basis of the stress response (activation of the HPA axis leading to production of cortisol) and its interactions with the immune and other endocrine systems. Discusses pathologies of the stress system.

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                                                                                                                                                                                                                                                    • Frenette, E., A. Lui, and M. Cao. 2012. Neurohormones and sleep. Vitamins and Hormones 89:1–17.

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                                                                                                                                                                                                                                                      Reviews the reciprocal interactions between sleep and various hormones including melatonin, growth hormone, cortisol, gonadal hormones, prolactin, thyroid hormones, vasopressin, oxytocin, leptin, and insulin.

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                                                                                                                                                                                                                                                      • Gómez-González, B., D. Domínguez-Salazar, G. Hurtado-Alvarado, et al. 2012. Role of sleep in the regulation of the immune system and the pituitary hormones. Annals of the New York Academy of Sciences 1261:97–106.

                                                                                                                                                                                                                                                        DOI: 10.1111/j.1749-6632.2012.06616.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                        Reviews the bidirectional relationship between sleep and pituitary hormones (hormones that trigger the release of hormones by peripheral glands such as the thyroid or gonads), as well as that between sleep and cytokines (substances related to inflammation).

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                                                                                                                                                                                                                                                        • Morris, C. J., D. Aeschbach, and F. A. Scheer. 2012. Circadian system, sleep and endocrinology. Molecular and Cellular Endocrinology 349:91–104.

                                                                                                                                                                                                                                                          DOI: 10.1016/j.mce.2011.09.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                          Reviews the basics of sleep and circadian rhythms and then focuses on the reciprocal interactions between sleep and various hormones including melatonin, cortisol, growth hormone, prolactin, thyroid hormones, insulin, ghrelin, and leptin. Discusses negative effects of circadian misalignment on cardio-metabolic health.

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                                                                                                                                                                                                                                                          • Pannain, S., and E. van Cauter. 2007. Modulation of endocrine function by sleep-wake homeostasis and circadian rhythmicity. Sleep Medicine Clinics 2:147–159.

                                                                                                                                                                                                                                                            DOI: 10.1016/j.jsmc.2007.03.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                            Reviews the effects of sleep on various hormonal systems, including that of growth hormone, cortisol, thyroid hormones, prolactin, and vasopressin. Includes helpful figures.

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                                                                                                                                                                                                                                                            • Steiger, A. 2003. Sleep and endocrine regulation. Frontiers in Bioscience 8:s358–376.

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                                                                                                                                                                                                                                                              A broad review of the interactions between sleep EEG and various endocrine systems. With regard to sleep, covers the basic activity of each hormone system, the effects of hormonal disorders, the effects of hormone administration, and findings from animal models of endocrine disorders.

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                                                                                                                                                                                                                                                              Sleep and Insulin/Glucose Regulation

                                                                                                                                                                                                                                                              Especially important are the endocrine disturbances shown to accompany insufficient sleep including alterations in metabolic hormones such as ghrelin—a hormone that promotes appetite—and leptin—a hormone that suppresses appetite (van Cauter, et al. 2007, Leproult and Van Cauter 2010). Behaviorally, sleep disruption leads to a dysregulation of appetite (St-Onge 2013) that contributes to disruption of glucose regulation at the physiological level (Spiegel, et al. 2009) and ultimately to increased risk of metabolic diseases such as obesity, insulin resistance, metabolic syndrome and Type II Diabetes (Knutson, et al. 2007).

                                                                                                                                                                                                                                                              • Knutson, K. L., K. Spiegel, P. Penev, and E. Van Cauter. 2007. The metabolic consequences of sleep deprivation. Sleep Medicine Reviews 11:163–178.

                                                                                                                                                                                                                                                                DOI: 10.1016/j.smrv.2007.01.002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                Reviews findings of epidemiologic and laboratory studies regarding the mechanisms by which sleep loss may increase risk for diabetes and obesity. Specifically covers the effects of sleep loss on alterations in glucose metabolism, up-regulation of appetite, and decreased energy expenditure. Includes some figures and tables.

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                                                                                                                                                                                                                                                                • Leproult, R., and E. Van Cauter. 2010. Role of sleep and sleep loss in hormonal release and metabolism. Endocrine Development 17:11–21.

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                                                                                                                                                                                                                                                                  Reviews findings from epidemiologic and laboratory studies indicating that sleep loss is associated with increased risk for obesity and diabetes.

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                                                                                                                                                                                                                                                                  • Spiegel, K., E. Tasali, R. Leproult, and E. Van Cauter. 2009. Effects of poor and short sleep on glucose metabolism and obesity risk. Nature Reviews Endocrinology 5:253–261.

                                                                                                                                                                                                                                                                    DOI: 10.1038/nrendo.2009.23Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                    Reviews findings from epidemiologic and laboratory studies regarding the effect of short and/or poor quality sleep on risk for diabetes and obesity. Additionally reviews the association between obstructive sleep apnea and diabetes, weight gain, and polycystic-ovary syndrome. Includes useful figures.

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                                                                                                                                                                                                                                                                    • St-Onge, M.- P. 2013. The role of sleep duration in the regulation of energy balance: Effects on energy intakes and expenditure. Journal of Clinical Sleep Medicine 9:73–80.

                                                                                                                                                                                                                                                                      DOI: 10.5664/jcsm.2348.Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                      Reviews the effects of total sleep deprivation, sleep restriction, and poor sleep quality on appetite and hunger, food intake, and energy expenditure. Discusses the effects of sleep loss on the energy regulatory hormones leptin and ghrelin.

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                                                                                                                                                                                                                                                                      • Van Cauter, E., U. Holmback, K. Knutson, et al. 2007. Impact of sleep and sleep loss on neuroendocrine and metabolic function. Hormone Research 67.1 (Suppl.): 2–9.

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                                                                                                                                                                                                                                                                        Reviews evidence for the links between sleep deprivation, metabolic changes, and obesity.

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                                                                                                                                                                                                                                                                        Sleep and the Hypothalamic-Pituitary-Adrenal (HPA) Axis

                                                                                                                                                                                                                                                                        The fluctuations in blood plasma cortisol (the end product of the HPA axis) follow a highly predictable pattern from a twenty-four-hour nadir (low point) during early slow wave sleep to a peak immediately following awakening (Kalsbeek, et al. 2012) during the cortisol awakening response—the sharp rise in cortisol that follows awakening (Elder, et al. 2014). This circadian rhythm of cortisol is distinct from the stress response whereby the HPA axis is activated and cortisol rises due to an acute or chronic stressor. As in the case of other neuroendocrine systems, insufficient sleep can alter these patterns leading to abnormalities such as elevated evening cortisol (Steiger 2006).

                                                                                                                                                                                                                                                                        • Elder, G. J., M. A. Wetherell, N. L. Barclay, and J. G. Ellis. 2014. The cortisol awakening response—applications and implications for sleep medicine. Sleep Medicine Reviews 18.3: 215–224.

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                                                                                                                                                                                                                                                                          A review of the cortisol awakening response (the sharp rise in cortisol that follows awakening) in terms of phenomenology, function, and relationship to disorders such as insomnia.

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                                                                                                                                                                                                                                                                          • Kalsbeek, A., R. van der Spek, J. Lei, E. Endert, R. M. Buijs, and E. Fliers. 2012. Circadian rhythms in the hypothalamo-pituitary-adrenal (HPA) axis. Molecular and Cellular Endocrinology 349.1: 20–29.

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                                                                                                                                                                                                                                                                            A comprehensive review of predictable circadian fluctuations in activity of the HPA axis in animals and humans.

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                                                                                                                                                                                                                                                                            • Steiger, A. 2006. Sleep and the hypothalamo-pituitary-adrenocortical system. Sleep Medicine Reviews 6.2: 125–138.

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                                                                                                                                                                                                                                                                              Reviews the literature regarding the reciprocal interaction between the sleep EEG and activity of the HPA axis and levels of cortisol. Additionally discusses animal models of altered HPA systems and the interactions between the HPA and other neuroendocrine systems.

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                                                                                                                                                                                                                                                                              Sleep and Gonadal Hormones

                                                                                                                                                                                                                                                                              Like cortisol, the hormone testosterone in males varies predictably from an evening nadir (low point) to a morning peak (Diver, et al. 2003). Gonadal hormones can, in turn, affect sleep and circadian rhythms (Mong, et al. 2011). The intimate relationship between gonadal hormones and sleep is also evidenced by the sleep disturbances arising at menopause in women that can be alleviated by estrogen replacement therapy (Moline, et al. 2003). Like estrogen in women, testosterone can interact with sleep disorders in males (Anderson and Tufik 2008).

                                                                                                                                                                                                                                                                              • Anderson, M. L., and S. Tufik. 2008. The effects of testosterone on sleep and sleep-disordered breathing in men: Its bidirectional interaction with erectile function. Sleep Medicine Reviews 12:365–379.

                                                                                                                                                                                                                                                                                DOI: 10.1016/j.smrv.2007.12.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                Reviews the reciprocal relationship between sleep and testosterone activity in men, as well as the effect of testosterone on sleep apnea and sleep-related erections.

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                                                                                                                                                                                                                                                                                • Diver, M. J., K. E. Imtiaz, A. M. Ahmad, J. P. Vora, and W. D. Fraser. 2003. Diurnal rhythms of serum total, free and bioavailable testosterone and of SHBG in middle-aged men compared with those in young men. Clinical Endocrinology 58:710–717.

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                                                                                                                                                                                                                                                                                  A recent example of a number of reports showing a clear circadian pattern in the secretion of testosterone that is similar to that of cortisol with a morning acrophase (peak point) and a nighttime nadir (low point).

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                                                                                                                                                                                                                                                                                  • Moline, M. L., L. Broch, R. Zak, and V. Gross. 2003. Sleep in women across the life cycle from adulthood through menopause. Sleep Medicine Reviews 7.2: 155–177.

                                                                                                                                                                                                                                                                                    DOI: 10.1053/smrv.2001.0228Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                    Reviews subjective and objective sleep parameters in women during the menstrual cycle, pregnancy and postpartum, peri-menopause, and menopause.

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                                                                                                                                                                                                                                                                                    • Mong, J. A., F. C. Baker, M. M. Mahoney, K. N. Paul, M. D. Schwartz, K. Semba, and R. Silver. 2011. Sleep, rhythms, and the endocrine brain: Influence of sex and gonadal hormones. Journal of Neuroscience 31.45: 16107–16116.

                                                                                                                                                                                                                                                                                      DOI: 10.1523/JNEUROSCI.4175-11.2011Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                      Reviews human and animal literature regarding the effects of sex and gonadal hormones on sleep and circadian rhythms, with an emphasis on ovarian hormones in females. Includes some figures.

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                                                                                                                                                                                                                                                                                      Sleep and Autonomic and Temperature Control Systems

                                                                                                                                                                                                                                                                                      The autonomic nervous system comprises the closely interacting sympathetic activating (“fight or flight”) and restorative parasympathetic (“rest and digest”) systems. The onset of NREM sleep is associated with a marked decrease in sympathetic and increase in parasympathetic drive that is expressed to the greatest extent during SWS (Trinder, et al. 2001, Trinder, et al. 2012). Sympathetic drive increases during REM but not to the levels seen in waking (Trinder, et al. 2001, Trinder, et al. 2012). These changes affect cardiovascular function leading to predictable decline in blood pressure during sleep in healthy individuals (Silvani 2008). Likewise, sleep disruption can lead to alterations in cardiovascular control (Smolensky, et al. 2007; Stein and Pu 2012). Sleep and temperature co-vary in a predictable manner across the night and disruption in one can lead to disruption of the other (Krauchi and Deboer 2010; Van Someren 2006).

                                                                                                                                                                                                                                                                                      • Krauchi, K., and T. Deboer. 2010. The interrelationship between sleep regulation and thermoregulation. Frontiers in Bioscience 15:604–625.

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                                                                                                                                                                                                                                                                                        Reviews the relationship between sleep and temperature rhythms under normal conditions as well as under experimental manipulations in humans and rats. Includes figures.

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                                                                                                                                                                                                                                                                                        • Silvani, A. 2008. Physiological sleep-dependent changes in arterial blood pressure: Central autonomic commands and baroreflex control. Clinical and Experimental Pharmacology and Physiology 35:987–994.

                                                                                                                                                                                                                                                                                          DOI: 10.1111/j.1440-1681.2008.04985.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                          Reviews the contributions of central (forebrain) and reflex (occurring below the forebrain) mechanisms to measures of arterial blood pressure during NREM and REM sleep stages. Additionally reviews cardiac variables such as cardiac output and vascular conductance during these sleep stages.

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                                                                                                                                                                                                                                                                                          • Smolensky, M. H., R. C. Hermida, R. J. Castriotta, and F. Portaluppi. 2007. Role of sleep-wake cycle on blood pressure circadian rhythms and hypertension. Sleep Medicine 8:668–680.

                                                                                                                                                                                                                                                                                            DOI: 10.1016/j.sleep.2006.11.011Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                            Reviews the daily variation in blood pressure, including its underlying mechanisms as well as the role of sleep, and sleep-related changes in this rhythm.

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                                                                                                                                                                                                                                                                                            • Stein, P. K., and Y. Pu. 2012. Heart rate variability, sleep, and sleep disorders. Sleep Medicine Reviews 16:47–66.

                                                                                                                                                                                                                                                                                              DOI: 10.1016/j.smrv.2011.02.005Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                              Reviews changes in heart rate variability (HRV) across different sleep stages, the effects of various sleep problems on HRV, and the applications of using HRV as a clinical marker. HRV measures changes in heart rate over time which can indicate the relative strength of sympathetic and parasympathetic drive. Includes tables and some figures.

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                                                                                                                                                                                                                                                                                              • Trinder, J., J. Kleiman, M. Carrington, et al. 2001. Autonomic activity during human sleep as a function of time and sleep stage. Journal of Sleep Research 10.4: 253–264.

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                                                                                                                                                                                                                                                                                                Original report characterizing heart rate, blood pressure, and cardiac sympathetic activity over the course of sleep.

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                                                                                                                                                                                                                                                                                                • Trinder, J., J. Waloszek, M. J. Woods, and A. S. Jordan. 2012. Sleep and cardiovascular regulation. Pflugers Archive – European Journal of Physiology 463:161–168.

                                                                                                                                                                                                                                                                                                  DOI: 10.1007/s00424-011-1041-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                  Reviews the bidirectional relationship between sleep and cardiovascular activity, as well as the effect of arousals and sleep disturbances on cardiovascular functioning.

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                                                                                                                                                                                                                                                                                                  • Van Someren, E. J. W. 2006. Mechanisms and functions of coupling between sleep and temperature rhythms. Progress in Brain Research 153:309–324.

                                                                                                                                                                                                                                                                                                    DOI: 10.1016/S0079-6123(06)53018-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                    Reviews the relationship between sleep and body temperature rhythms, as well as the possible mechanisms underlying this relationship. Discusses evidence for a non-thermoregulatory function of increased blood flow to the skin during sleep. Includes figures.

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                                                                                                                                                                                                                                                                                                    Sleep and Human Performance

                                                                                                                                                                                                                                                                                                    Like other aspects of human physiology and behavior, sleep need varies widely across individuals (Aeschbach, et al. 2001). The interaction of sleep duration with longevity is a topic of much epidemiological interest as well as much controversy (Kripke, et al. 2002). In addition to the negative metabolic and neuroendocrine effects of insufficient sleep, the effects of insufficient sleep on vigilance and performance are of immense significance to public health (Mitler, et al. 1997). Sleepiness is an often-identified factor in automobile, public transportation, and industrial accidents (e.g., Garbarino, et al. 2001).

                                                                                                                                                                                                                                                                                                    • Aeschbach, D., T. T. Postolache, L. Sher, J. R. Matthews, M. A. Jackson, and T. A. Wehr. 2001. Evidence from the waking electroencephalogram that short sleepers live under higher homeostatic sleep pressure than long sleepers. Neuroscience 102:493–502.

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                                                                                                                                                                                                                                                                                                      First evidence of a biological basis for the difference between short and long sleepers.

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                                                                                                                                                                                                                                                                                                      • Garbarino, S., L. Nobili, M. Beelke, F. De Carli, F. Ferrillo. 2001. The contributing role of sleepiness in highway vehicle accidents. Sleep 24:203–206.

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                                                                                                                                                                                                                                                                                                        Clear population-wise demonstration of the link between sleepiness and motor vehicle accidents.

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                                                                                                                                                                                                                                                                                                        • Kripke, D. F., L. Garfinkle, D. L. Wingard, M. R. Klauber, and M. R. Marler. 2002. Mortality associated with sleep duration and insomnia. Archives of General Psychiatry 59:131–136.

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                                                                                                                                                                                                                                                                                                          Controversial finding that long (eight-plus hours) and short (six-minus hours) sleep durations are associated with increased mortality compared to approximately seven hours.

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                                                                                                                                                                                                                                                                                                          • Mitler, M. M., J. C. Miller, J. J. Lipsitz, J. K. Walsh, and C. D. Wylie. 1997. The sleep of long haul truck drivers. The New England Journal of Medicine 337:755–761.

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                                                                                                                                                                                                                                                                                                            Demonstration that truck drivers actually enter N1 sleep while driving when overtired.

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                                                                                                                                                                                                                                                                                                            Sleep Across the Lifespan

                                                                                                                                                                                                                                                                                                            The timing, duration, and architecture of sleep undergo dramatic changes during the first few years of life. More subtle but physiologically and cognitively significant changes in sleep quality, architecture, and circadian timing take place at puberty, middle age, and in later life as reviewed by Crabtree and Williams 2009 and Ohayon, et al. 2004. Neonates sleep in bouts of several hours and REM sleep can occupy as much as half of this time, with even higher percentages believed to occur in utero, but then declining to an adult-like 20–25 percent by age four.

                                                                                                                                                                                                                                                                                                            • Crabtree, V. M., and N. A. Williams. 2009. Normal sleep in children and adolescents. Child and Adolescent Psychiatric Clinics of North America 18:799–811.

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                                                                                                                                                                                                                                                                                                              Accessible overview of literature regarding changes in sleep behavior and sleep architecture in children 0–18 years old. Covers findings of gender differences. Discusses measurement techniques and includes tables of sleep data compiled from multiple studies.

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                                                                                                                                                                                                                                                                                                              • Ohayon, M., M. A. Carskadon, C. Guilleminault, and M. V. Vitiello. 2004. Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals: Developing normative sleep values across the human lifespan. Sleep 27:1255–1273.

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                                                                                                                                                                                                                                                                                                                Meta-analysis of sixty-five studies investigating objectively measured sleep variables in subjects 5–102 years old. Specifically focuses on sleep latency, sleep efficiency, total sleep time, sleep stage composition, and time awake after sleep onset. Discusses age trends, sex trends, and the impact of moderator variables. Includes figures of results. (Meta-analyses statistically combine multiple published studies carried out on a certain population or phenomenon and draw more general conclusions about their combined findings.)

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                                                                                                                                                                                                                                                                                                                Infancy

                                                                                                                                                                                                                                                                                                                Infancy is the time in life during which striking changes in sleep occur over periods on the order of months to years (Davis, et al. 2004; Henderson, et al. 2011). One of the most well known changes over early infancy is the consolidation of sleep from multiple bouts into a single nocturnal episode often with a diurnal nap (Galland, et al. 2012; Heraghty, et al. 2008). These changes are undoubtedly linked to the similarly dramatic reorganization of brain circuits during this same period in life.

                                                                                                                                                                                                                                                                                                                • Davis, K. F., K. P. Parker, and G. L. Montgomery. 2004. Sleep in infants and young children: Part one: Normal sleep. Journal of Pediatric Health Care 18:65–71.

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                                                                                                                                                                                                                                                                                                                  Accessible review of changes in sleep behavior and sleep architecture in children from birth to five years of age. Includes discussion of sleep promoting strategies, risks of co-sleeping, and cultural and economic influences on sleep practices.

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                                                                                                                                                                                                                                                                                                                  • Galland, B. C., B. J. Taylor, D. E. Elder, and P. Herbison. 2012. Normal sleep patterns in infants and children: A systematic review of observational studies. Sleep Medicine Reviews 16:213–222.

                                                                                                                                                                                                                                                                                                                    DOI: 10.1016/j.smrv.2011.06.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                    Meta-analysis of thirty-four studies investigating subjectively measured sleep duration, night awakenings, sleep latency, longest overnight sleep period, and number of daytime naps in children —from birth to twelve years old. Includes figures of results.

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                                                                                                                                                                                                                                                                                                                    • Henderson, J. M. T., K. G. France, and N. M. Blampied. 2011. The consolidation of infants’ nocturnal sleep across the first year of life. Sleep Medicine Reviews 15:211–220.

                                                                                                                                                                                                                                                                                                                      DOI: 10.1016/j.smrv.2010.08.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                      Meta-analysis of twenty-six studies investigating changes in sleep patterns in infants from birth to twelve months old. Specifically focuses on the longest sustained sleep period, longest self-regulated sleep period, and sleeping through the night.

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                                                                                                                                                                                                                                                                                                                      • Heraghty, J. L., T. N. Hilliard, A. J. Henderson, and P. J. Fleming. 2008. The physiology of sleep in infants. Archives of Diseases in Childhood 93:982–985.

                                                                                                                                                                                                                                                                                                                        DOI: 10.1136/adc.2006.113290Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                        A brief review of sleep development and its interactions with autonomic physiology and endocrine systems in infants.

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                                                                                                                                                                                                                                                                                                                        Adolescence

                                                                                                                                                                                                                                                                                                                        Reorganization and pruning (naturally occurring reduction in the number of neurons or synapses) of brain circuitry takes place around puberty and, as in infancy, sleep changes at this time may reflect or facilitate these changes. The most prominent change observed is a phase delay (shift to a later time) in the circadian timing of sleep (Carskadon, et al. 1998; Colrain and Baker 2011) that goes along with changes in the homeostatic regulation of sleep (Hagenauer, et al. 2009). These changes have profound implications for adolescents’ ability to achieve sufficient sleep and can contribute to both academic and emotional problems (Carskadon 2011). Sleep disorders such as insomnia and delayed sleep phase disorder often arise during adolescence (Gradisar, et al. 2011). Both similarities and differences in sleep changes across adolescence are observed in different cultures (Olds, et al. 2010).

                                                                                                                                                                                                                                                                                                                        • Carskadon, M. 2011. Sleep in adolescents: The perfect storm. Pediatric Clinics of North America 58:637–647.

                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/j.pcl.2011.03.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                          Accessible review of sleep patterns and sleep regulation in adolescents. Discusses psychosocial factors affecting sleep behavior and negative outcomes of insufficient sleep common during this age period.

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                                                                                                                                                                                                                                                                                                                          • Carskadon, M. A., A. R. Wolfson, C. Acebo, O. Tzischinsky, and R. Siefer. 1998. Adolescent sleep patterns, circadian timing, and sleepiness at a transition to earlier days. Sleep 21:871–881.

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                                                                                                                                                                                                                                                                                                                            Early demonstration that circadian phase delay and, hence, later sleep timing normally occurs around puberty.

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                                                                                                                                                                                                                                                                                                                            • Colrain, I. M., and F. C. Baker. 2011. Changes in sleep as a function of adolescent development. Neuropsychology Review 21:5–21.

                                                                                                                                                                                                                                                                                                                              DOI: 10.1007/s11065-010-9155-5Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                              Reviews pubertal and brain development and changes in sleep behavior, regulation, and architecture. Additionally reviews and discusses several negative effects of inadequate sleep in this age group, including those on mental health, school performance, and alcohol/substance use. Includes figures.

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                                                                                                                                                                                                                                                                                                                              • Gradisar, M., G. Gardner, and H. Dohnt. 2011. Recent worldwide sleep patterns and problems during adolescence: A review and meta-analysis of age, region, and sleep. Sleep Medicine 12:110–118.

                                                                                                                                                                                                                                                                                                                                DOI: 10.1016/j.sleep.2010.11.008Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                Meta-analysis of forty-one studies conducted in North American, European, and Asian countries. Compiles results regarding sleep onset, total sleep, and wake times on school days and weekends. Also reviews and compiles data regarding daytime sleepiness, insomnia, and delayed-sleep disorder. Includes figures of results.

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                                                                                                                                                                                                                                                                                                                                • Hagenauer, M. H., J. I. Perryman, T. M. Lee, and M. A. Carskadon. 2009. Adolescent changes in the homeostatic and circadian regulation of sleep. Developmental Neuroscience 32:276–284.

                                                                                                                                                                                                                                                                                                                                  DOI: 10.1159/000216538Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                  Reviews human and animal literature pointing to a delayed sleep phase in adolescent mammals. Focuses on potential mechanisms underlying this shift in sleep phase. Includes figures.

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                                                                                                                                                                                                                                                                                                                                  • Olds, T., S. Blunden, J. Petkov, and F. Forchino. 2010. The relationships between sex, age, geography and time in bed in adolescents: A meta-analysis of data from 23 countries. Sleep Medicine Review 14:371–378.

                                                                                                                                                                                                                                                                                                                                    DOI: 10.1016/j.smrv.2009.12.002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                    Meta-analysis of thirty studies investigating sleep duration in children nine to eighteen years of age. Reports effects of age, sex, world region, and day type (weekend or school day). Includes figures of results.

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                                                                                                                                                                                                                                                                                                                                    Aging and Senescence

                                                                                                                                                                                                                                                                                                                                    The most notable change in sleep architecture taking place with aging is a steep decline in SWS beginning in middle age (Pace-Schott and Spencer 2011). More subtle changes in the quality and circadian timing of sleep (phase advance) begin at this time and become increasingly prominent in older adults (Pace-Schott and Spencer 2011). The prevalence of sleep disorders increases with aging (Crowley 2011). However, multiple factors extrinsic to aging per se impact sleep quality in elderly adults including medical conditions, medication effects, and psychosocial influences (Ancoli-Israel 2009; Stepnowsky and Ancoli-Israel 2008.

                                                                                                                                                                                                                                                                                                                                    • Ancoli-Israel, S. 2009. Sleep and its disorders in aging populations. Sleep Medicine 10:S7–S11.

                                                                                                                                                                                                                                                                                                                                      DOI: 10.1016/j.sleep.2009.07.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                      Reviews literature regarding sleep quality in the elderly and the effects of poor sleep on health and physical and cognitive functioning. Additionally reviews sleep problems related to disease and medication use in this age group.

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                                                                                                                                                                                                                                                                                                                                      • Crowley, K. 2011. Sleep and sleep disorders in older adults. Neuropsychology Review 21:41–53.

                                                                                                                                                                                                                                                                                                                                        DOI: 10.1007/s11065-010-9154-6Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                        Provides an overview of changes in sleep and circadian rhythms occurring with healthy aging, as well as primary sleep disorders and sleep problems associated with neurological disorders in the elderly. Discusses consequences of poor sleep and potential mechanisms underlying age-related changes in sleep.

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                                                                                                                                                                                                                                                                                                                                        • Pace-Schott, E. F., and R. M. Spencer. 2011. Age-related changes in the cognitive function of sleep. Progress in Brain Research 191:75–89.

                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/B978-0-444-53752-2.00012-6Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                          A thorough review of changes in sleep occurring with healthy aging. Covers changes in sleep behavior, architecture, and regulation processes. Additionally reviews age-related changes in memory systems and the interactions between sleep and memory with advancing age.

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                                                                                                                                                                                                                                                                                                                                          • Stepnowsky, C. J., Jr., and S. Ancoli-Israel. 2008. Sleep and its disorders in seniors. Journal of Clinical Sleep Medicine 3.2: 281–293.

                                                                                                                                                                                                                                                                                                                                            DOI: 10.1016/j.jsmc.2008.01.011Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                            Reviews age-related changes in sleep and circadian rhythms from a clinical perspective. Covers sleep disorders and sleep problems associated with medical and psychiatric illnesses. Discusses treatments and interventions.

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                                                                                                                                                                                                                                                                                                                                            Phylogeny of Sleep

                                                                                                                                                                                                                                                                                                                                            The vast majority of animals show rest-activity cycles that are influenced by circadian rhythms which can be entrained (synchronized) to ambient light-dark and other environmental cycles. A behavioral state clearly identifiable as sleep exists in all homeotherms (“warm-blooded” animals; Phylogeny of Sleep website). REM sleep occurs only in homeotherms, and most clearly in mammals, including the primitive monotremes (Phylogeny of Sleep). However, recent research increasingly identifies sleep-like states in poikilothermic (“cold-blooded”) vertebrates as well as many invertebrates (Phylogeny of Sleep). Remarkable patterns of unihemispheric sleep (one brain hemisphere sleeping at a time) occur in cetaceans (whales and dolphins), some seals and birds (Rattenborg, et al. 2000; Rattenborg 2006; Siegel 2008). These findings, along with physiological studies in humans, animals, and even in cell cultures and brain slices, have contributed to the growing interest in “local sleep” (Rattenborg, et al. 2012). Sleep is a flexible behavior and clearly has responded to evolutionary demands in different organisms (Siegel 2009). It is important to note that sleep is one of several inactive behavioral states in animals such as hibernation, estivation (dormancy during hot season), and torpor (a drowsy, inactive state) that may or may not be physiologically or evolutionarily analogous to sleep (Siegel 2008).

                                                                                                                                                                                                                                                                                                                                            • Phylogeny of Sleep.

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                                                                                                                                                                                                                                                                                                                                              A user-friendly website under the domain of Boston University containing informational resources including a searchable database of sleep characteristics across different species.

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                                                                                                                                                                                                                                                                                                                                              • Rattenborg, N. C. 2006. Do birds sleep in flight? Naturwissenschaften 93.9: 413–425.

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                                                                                                                                                                                                                                                                                                                                                An exploration of the possibility that unihemispheric sleep may occur in birds even during flight.

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                                                                                                                                                                                                                                                                                                                                                • Rattenborg, N. C., C. J. Amlaner, and S. L. Lima. 2000. Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep. Neuroscience and Biobehavioral Reviews 24.8: 817–842.

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                                                                                                                                                                                                                                                                                                                                                  A review of the phylogeny and possible evolutionary significance of the remarkable phenomenon of unihemispheric sleep in birds and cetaceans.

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                                                                                                                                                                                                                                                                                                                                                  • Rattenborg, N. C., S. L. Lima, and J. A. Lesku. 2012. Sleep locally, act globally. Neuroscientist 18.5: 533–546.

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                                                                                                                                                                                                                                                                                                                                                    A discussion combining ethological (animal behavior) findings with the emerging science of local sleep.

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                                                                                                                                                                                                                                                                                                                                                    • Siegel, J. M. 2008. Do all animals sleep? Trends in Neuroscience 31.4: 208–213.

                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1016/j.tins.2008.02.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                      A review of the diversity of sleep and sleep-like behaviors among different phyla. Argues that sleep, rather than being a unitary state with a primary function, has evolved for a variety of evolutionary purposes.

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                                                                                                                                                                                                                                                                                                                                                      • Siegel, J. M. 2009. Sleep viewed as a state of adaptive inactivity. Nature Reviews Neuroscience 10:747–753.

                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1038/nrn2697Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                        Further exploration of the hypothesis that sleep evolved to fulfill a variety of ecological functions by which a period of inactivity might promote survival of a species.

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                                                                                                                                                                                                                                                                                                                                                        Sleep Disorders

                                                                                                                                                                                                                                                                                                                                                        Major categories of sleep disorders include insomnias, which are characterized by problems initiating and/or maintaining sleep; hypersomnias (e.g., narcolepsy), characterized by excessive daytime sleepiness; sleep-related breathing disorders, such as sleep apnea; parasomnias, characterized by abnormal behaviors during sleep (e.g., sleep walking, nightmares, and bedwetting); circadian rhythm sleep disorders, which involve disruption in the timing of sleep; and sleep-related movement disorders, such as restless legs syndrome. As noted in Textbooks and Reference Volumes, many medical textbooks deal with sleep disorders in general, and there are now numerous specific volumes addressing each disorder and its associated conditions separately. Sleep is a rapidly developing specialty in medicine. The identification of linkages between sleep disorders and other medical conditions has important health implications, among the most prominent being the link between sleep apnea and cardiovascular disease (e.g., Marin, et al. 2012). Excellent overviews on sleep disorders are available for the general practitioner (e.g., Ramar and Olson 2013; Thorpy 2012), the neurologist (Vaughn 2012; Voderholzer and Guilleminault 2012), the pediatrician (Hiscock and Davey 2012) and the circadian specialist (Zhu and Zee 2012). In addition, there are now reviews of technologies used to diagnose and treat sleep disorders (Bianchi and Thomas 2013) as well pharmacological interventions for these disorders (Ellenbogen and Pace-Schott 2012). In addition to the references that follow, the reader is referred to the textbooks, journals (e.g., Sleep Medicine Reviews), and resource volumes in the other sections of this bibliography.

                                                                                                                                                                                                                                                                                                                                                        • Bianchi, M. T., and R. J. Thomas. 2013. Technical advances in the characterization of the complexity of sleep and sleep disorders. Progress in Neuro-Psychopharmacology & Biological Psychiatry 45:277–286.

                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/j.pnpbp.2012.09.017Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                          Reviews both standard and alternative techniques used to measure and analyze sleep. Discusses implications for studying sleep disorders. Includes some figures.

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                                                                                                                                                                                                                                                                                                                                                          • Ellenbogen, J. M., and E. F. Pace-Schott. 2012. Drug-induced sleep: Theoretical and practical considerations. Pflugers Archiv 463.1: 177–186.

                                                                                                                                                                                                                                                                                                                                                            DOI: 10.1007/s00424-011-1033-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                            Reviews pharmacology of sleep medications targeting the major categories of sleep disorders. Discusses issues regarding evaluation of drug-induced sleep.

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                                                                                                                                                                                                                                                                                                                                                            • Hiscock, H., and M. J. Davey. 2012. Sleep disorders in infants and children. Journal of Paediatrics and Child Health (16 December).

                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1111/jpc.12033Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                              Brief review of common sleep problems and management strategies in infants and young children. Written from a clinical perspective.

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                                                                                                                                                                                                                                                                                                                                                              • Marin, J. M., A. Agusti, I. Villar, et al. 2012. Association between treated and untreated obstructive sleep apnea and risk of hypertension. JAMA 307:2169–2176.

                                                                                                                                                                                                                                                                                                                                                                DOI: 10.1001/jama.2012.3418Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                One of many empirical studies showing increased independent risk of hypertension in persons with obstructive sleep apnea that can be lowered by continuous positive airway (CPAP) treatment.

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                                                                                                                                                                                                                                                                                                                                                                • Ramar, K., and E. J. Olson. 2013. Management of common sleep disorders. American Family Physician (15 August) 88.4: 231–238.

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                                                                                                                                                                                                                                                                                                                                                                  Reviews sleep disorders and their treatments from a clinical perspective. Includes useful tables.

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                                                                                                                                                                                                                                                                                                                                                                  • Thorpy, M. J. 2012. Classification of sleep disorders. Neurotherapeutics 9.4: 687–701.

                                                                                                                                                                                                                                                                                                                                                                    DOI: 10.1007/s13311-012-0145-6Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                    Characterizes the many sleep disorders and their classification in the second edition of the International Classification of Sleep Disorders. Includes helpful tables.

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                                                                                                                                                                                                                                                                                                                                                                    • Vaughn, B. V. 2012. Sleep disorders. Preface. Neurologic Clinics 30.4: xiii–xiv.

                                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1016/j.ncl.2012.08.020Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                      Highlights the relationship between sleep and neurological disorders and, thus, the importance of incorporating sleep medicine into the practice of neurology. Introduces articles geared toward educating neurologists about these issues.

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                                                                                                                                                                                                                                                                                                                                                                      • Voderholzer, U., and C. Guilleminault. 2012. Sleep disorders. Handbook of Clinical Neurology 106:527–540.

                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1016/B978-0-444-52002-9.00031-0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                        Reviews sleep disorders including their clinical features, epidemiology, etiology, and treatment.

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                                                                                                                                                                                                                                                                                                                                                                        • Zhu, L., and P. C. Zee. 2012. Circadian rhythm sleep disorders. Neurologic Clinics 30.4: 1167–1191.

                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/j.ncl.2012.08.011Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                          Reviews circadian rhythm biology and common circadian rhythm sleep disorders including their pathophysiology, clinical features, diagnosis, and treatment. Includes figures.

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                                                                                                                                                                                                                                                                                                                                                                          Recent Findings

                                                                                                                                                                                                                                                                                                                                                                          New discoveries in sleep medicine are reported with increasing frequency. For example, the identification of orexin deficiency as the physiological basis of narcolepsy has revolutionized our understanding of this disorder of excessive sleepiness and sleep attacks (Chemelli, et al. 1999; Lin, et al. 1999; Thannickal, et al. 2000). With the advent of advanced genetic screening techniques such as genome-wide association studies, many new discoveries are likely imminent, such as the discovery of an inherited form of advanced sleep phase syndrome (Reid, et al. 2001).

                                                                                                                                                                                                                                                                                                                                                                          Theories of Dreaming

                                                                                                                                                                                                                                                                                                                                                                          As an intriguing and observable aspect of human experience available to all, interest in the nature and origin of dreams has existed for millennia. With recent advances in neuroscience, a number of preexisting cognitive and psychodynamic theories have been reworked and new theories have emerged.

                                                                                                                                                                                                                                                                                                                                                                          Reward-Based Theories

                                                                                                                                                                                                                                                                                                                                                                          Structures in the reward circuitry of the brain are activated during REM sleep and reward-seeking behaviors are a very common aspect of dream phenomenology (Perogamvros and Schwartz 2012; Perogamvros, et al. 2013). The latter observation has led dream theorists from Freud onward to speculate that dreaming occurs when goal or desire-seeking becomes initiated during sleep (Solms 1997).

                                                                                                                                                                                                                                                                                                                                                                          • Perogamvros, L, T. T. Dang-Vu, M. Desseilles, and S. Schwartz. 2013. Sleep and dreaming are for important matters. Frontiers in Psychology 4:474.

                                                                                                                                                                                                                                                                                                                                                                            DOI: 10.3389/fpsyg.2013.00474Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                            Reviews evidence of activation of emotion and reward systems during sleep and dreaming. Discusses the role this activation may play in memory consolidation, emotion regulation, social cognition, and creativity. Discusses the relationship between sleep disturbances and mood disorders.

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                                                                                                                                                                                                                                                                                                                                                                            • Perogamvros, L., and S. Schwartz. 2012. The roles of the reward system in sleep and dreaming. Neuroscience & Biobehavioral Reviews 36.8: 1934–1951.

                                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1016/j.neubiorev.2012.05.010Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                              Introduces the reward activation model of sleep and dreaming. Discusses how activation of the reward system during sleep could contribute to memory consolidation, modulation of REM sleep, and generation of dreams. Includes helpful figures.

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                                                                                                                                                                                                                                                                                                                                                                              • Solms, M. 1997. The neuropsychology of dreams: A clinico-anatomical study. Mahwah, NJ: Lawrence Erlbaum Associates.

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                                                                                                                                                                                                                                                                                                                                                                                An analysis of an extensive series of clinical cases as well as review of historical reports and scientific literature identifying the sites at which brain damage affects dream experience. Syndromes of dream loss or alteration are identified and a classification scheme proposed. Emphasis is placed on the effects of lesions or hyper-excitation on reward circuitry in describing the possible neural basis of normal dreaming.

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                                                                                                                                                                                                                                                                                                                                                                                Offline Memory Processing Theories

                                                                                                                                                                                                                                                                                                                                                                                The striking finding that sleep contributes to memory consolidation (the strengthening of memories that allows them to persist over time) along with the prominent appearance of recent waking experience in dreams (“day residue”) has led investigators to speculate that dreaming may be the conscious experience of such memory consolidation processes taking place during sleep (Stickgold, et al. 2001; Wamsley and Stickgold 2011).

                                                                                                                                                                                                                                                                                                                                                                                Default Mode Theories

                                                                                                                                                                                                                                                                                                                                                                                These theories are informed by the discovery of a network of brain regions that activate when attention is withdrawn from the external environment and focus shifts to self-related concerns, autobiographical memory and “daydreaming”—the so-called default-mode network. The overlap of some portions of this network with those that reactivate in REM sleep following their relative quiescence in NREM sleep have led some to speculate that dreaming reflects activity of this network, but in a manner different from its operation during waking reverie (Domhoff 2011; Pace-Schott 2013).

                                                                                                                                                                                                                                                                                                                                                                                Activation-Synthesis Hypothesis

                                                                                                                                                                                                                                                                                                                                                                                The Activation-Synthesis Hypothesis theory of dreaming (Hobson and McCarley 1977) was the first explicitly biological theory of dreaming based on the neurophysiology of sleep in animals and humans This theory, and its later formulation as the Activation-Input/Output Gating-Modulation (AIM) model (Hobson, et al. 2000), suggests that endogenous activation of the forebrain by brainstem structures during REM sleep is interpreted as true sensory input triggering hallucinations and that dreaming involves synthesis of this fictive (imaginary) input with narrative content and emotional concerns. This theory recently has been expanded to include functional theories of dreaming (Hobson 2009).

                                                                                                                                                                                                                                                                                                                                                                                • Hobson, J. A. 2009. REM sleep and dreaming: Towards a theory of protoconsciousness. Nature Reviews Neuroscience 10:803–813.

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                                                                                                                                                                                                                                                                                                                                                                                  Reviews developmental and evolutionary aspects of REM sleep, functional theories of this sleep stage, and the AIM model of behavioral state control. Presents a theory that REM sleep is a “protoconscious” state, a functional precursor to the development of waking consciousness.

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                                                                                                                                                                                                                                                                                                                                                                                  • Hobson, J. A., and R. W. McCarley. 1977. The brain as a dream-state generator: An activation-synthesis hypothesis of the dream process. American Journal of Psychiatry 134:1335–1348.

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                                                                                                                                                                                                                                                                                                                                                                                    Exposition of the Activation-Synthesis model of dreaming. Based on the contemporary neurophysiologic evidence, this model challenged the Freudian psychoanalytic theory of dreaming that dominated at the time.

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                                                                                                                                                                                                                                                                                                                                                                                    • Hobson, J. A., E. F. Pace-Schott, and R. Stickgold. 2000. Dreaming and the brain: Toward a cognitive neuroscience of conscious states. Behavioral and Brain Sciences 23:793–842.

                                                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1017/S0140525X00003976Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                      Exposition of the Activation-Synthesis and AIM models incorporating the additional information on regional brain activity during REM and NREM sleep provided by the PET studies of the late 1990s.

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                                                                                                                                                                                                                                                                                                                                                                                      Other Theories

                                                                                                                                                                                                                                                                                                                                                                                      Many other theories on the brain bases of dreaming have been advanced in the past several decades, including those based on self-organization and chaos theory (Kahn 2013), emotional regulation by the REM activation network (Levin and Nielsen 2007), evolutionary psychology such as threat rehearsal (Revonsuo 2000), and neurological disconnection syndromes (Schwartz and Maquet 2002).

                                                                                                                                                                                                                                                                                                                                                                                      • Kahn, D. 2013. Brain basis of self: Self-organization and lessons from dreaming. Frontiers in Psychology 4:408.

                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.3389/fpsyg.2013.00408Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                        Presents a theory positing that dreams consist of loosely connected, self-organized memories and that this self-organization (as opposed to organization imposed by the waking brain) allows for novel experiences and thereby an expansion of the waking self.

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                                                                                                                                                                                                                                                                                                                                                                                        • Levin, R., and T. A. Nielsen. 2007. Disturbed dreaming, posttraumatic stress disorder, and affect distress: A review and neurocognitive model. Psychological Bulletin 133:482–528.

                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1037/0033-2909.133.3.482Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                          A unified theory of nightmares based upon an emotion regulatory function of the anterior paralimbic REM activation area (midline limbic and paralimbic areas activated during REM sleep) in normal sleep and its disturbance by stress and psychopathology.

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                                                                                                                                                                                                                                                                                                                                                                                          • Revonsuo, A. 2000. The reinterpretation of dreams: An evolutionary hypothesis of the function of dreaming. Behavioral and Brain Sciences 23:877–901.

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                                                                                                                                                                                                                                                                                                                                                                                            An evolutionary theory of dreaming (the “threat simulation hypothesis”) based upon the presumed need for rehearsal of threat avoidance behaviors in our ancestors.

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                                                                                                                                                                                                                                                                                                                                                                                            • Schwartz, S., and P. Maquet. 2002. Sleep imaging and the neuropsychological assessment of dreams. Trends in Cognitive Sciences 6:23–30.

                                                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1016/S1364-6613(00)01818-0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                              Relates dream phenomenology to that associated with various neuropsychological syndromes and brain damage. Suggests frontal release (hypoactivation of the frontal cortex) and executive dysfunction as key determinants of the unique phenomenology of dreaming.

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                                                                                                                                                                                                                                                                                                                                                                                              Reviews on the Neurobiology of Dreaming

                                                                                                                                                                                                                                                                                                                                                                                              Recent review articles have drawn upon emerging neuroimaging data to elaborate upon the Theories of Dreaming as well as on linkages and similarities between dreaming and other aspects of normal and abnormal psychology. Such reviews have been based around neuroimaging data (Desseilles, et al. 2010; Perogamvros, et al. 2013) as well as a combination of neuroimaging, neurochemistry and neuropsychology (Hobson, et al. 1998; Pace-Schott 2011a; Pace-Schott 2011b).

                                                                                                                                                                                                                                                                                                                                                                                              • Desseilles, M., T. T. Dang-Vu, V. Sterpenich, and S. Schwartz. 2010. Cognitive and emotional processes during dreaming: A neuroimaging view. Consciousness and Cognition 20:998–1008.

                                                                                                                                                                                                                                                                                                                                                                                                DOI: 10.1016/j.concog.2010.10.005Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                Reviews findings from neuroimaging studies of REM sleep. Discusses how observed brain activation patterns relate to dream phenomenology.

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                                                                                                                                                                                                                                                                                                                                                                                                • Hobson, J. A., R. Stickgold, and E. F. Pace-Schott. 1998. The neuropsychology of REM sleep dreaming. NeuroReport 9:R1–R14.

                                                                                                                                                                                                                                                                                                                                                                                                  DOI: 10.1097/00001756-199802160-00033Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                  Draws on animal and human studies in reviewing the neural basis of REM sleep and dreaming. Relates dream phenomenology to brain activity.

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                                                                                                                                                                                                                                                                                                                                                                                                  • Pace-Schott, E. F. 2011a. The neurobiology of dreaming. In Principles and practice of sleep medicine. 5th ed. Edited by M. H. Kryger, T. Roth, and W. C. Dement, 563–575. Philadelphia: Elsevier.

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                                                                                                                                                                                                                                                                                                                                                                                                    An overview of dream biology in light of data from neuroimaging, neurochemistry, quantitative EEG, and neuropsychiatry.

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                                                                                                                                                                                                                                                                                                                                                                                                    • Pace-Schott, E. F. 2011b. REM sleep and dreaming. In Rapid eye movement sleep: Regulation and function. Edited by B. N. Mallick, S. R. Pandi-Permual, R. W. McCarley, and A. R. Morrison. Cambridge, UK: Cambridge Univ. Press.

                                                                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1017/CBO9780511921179Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                      Reviews the relationships between dreaming and REM sleep physiology revealed by new findings in neuroimaging and quantitative EEG research.

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                                                                                                                                                                                                                                                                                                                                                                                                      • Perogamvros, L., T. T. Dang-Vu, M. Desseilles, and S. Schwartz. 2013. Sleep and dreaming are for important matters. Frontiers in psychology 4:474.

                                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.3389/fpsyg.2013.00474Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                        Reviews findings regarding activation of emotion and reward systems during sleep and dreaming. Discusses how this activation might relate to memory consolidation, emotion regulation, social cognition, and creativity. Discusses links between sleep disturbances and mood disorders.

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                                                                                                                                                                                                                                                                                                                                                                                                        Recent Findings on the Neurobiology of Dreaming

                                                                                                                                                                                                                                                                                                                                                                                                        In the past several decades, striking findings have emerged from neuroimaging, electrophysiology, and the cognitive neurosciences that have provided new insights into the biological basis of dreams. Among these has been a renewed interest in lucid dreaming as an opportunity to identify, with temporal precision, the brain events that accompany specific dream experiences (e.g., Dresler, et al. 2011). Studies of dream lucidity have shown that, as was previously suspected, lucidity involves reactivation of frontal areas normally deactivated in REM sleep (Dresler, et al. 2012; Voss, et al. 2009) and that, indeed, lucidity could be induced by stimulation of these areas (Voss, et al. 2014). Other exciting new findings have linked dreams to replay of waking events (Stickgold, et al. 2000) and actual dream-content-linked change in waking performance (Wamsley, et al. 2010). Gamma frequency oscillations were shown to occur in REM sleep (Llinas and Ribary 1993), and disrupted coherence of these rhythms has been linked to dream “bizarreness”—the odd, illogical, improbable, or impossible events experienced in dreams (Corsi-Cabrera, et al. 2003).

                                                                                                                                                                                                                                                                                                                                                                                                        • Corsi-Cabrera, M., E. Miro, Y. Del-Rio-Portilla, E. Perez-Garci, Y. Villanueva, and M. A. Guevara. 2003. Rapid eye movement sleep dreaming is characterized by uncoupled EEG activity between frontal and perceptual cortical regions. Brain and Cognition 51:337–345.

                                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/S0278-2626(03)00037-XSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                          Electrophysiological evidence of reduced frontal influence on the posterior cortex during REM sleep—a possible basis for dream bizarreness.

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                                                                                                                                                                                                                                                                                                                                                                                                          • Dresler, M., S. P. Koch, R. Wehrle, et al. 2011. Dreamed movement elicits activation in the sensorimotor cortex. Current Biology 21:1833–1837.

                                                                                                                                                                                                                                                                                                                                                                                                            DOI: 10.1016/j.cub.2011.09.029Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                            Demonstration that specific dream content can be imaged using fMRI in lucid dreamers who “signal out” of their dream using eye movements when they are performing a specific task they were instructed, before sleep, to perform while they are dreaming.

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                                                                                                                                                                                                                                                                                                                                                                                                            • Dresler, M., R. Wehrle, V. I. Spoormaker, et al. 2012. Neural correlates of dream lucidity obtained from contrasting lucid versus non-lucid REM sleep: A combined EEG/fMRI case study. Sleep 35:1017–1020.

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                                                                                                                                                                                                                                                                                                                                                                                                              A neuroimaging study that showed reactivation of many of the regions that normally remain deactivated in non-lucid REM sleep.

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                                                                                                                                                                                                                                                                                                                                                                                                              • Llinas, R., and U. Ribary. 1993. Coherent 40-Hz oscillation characterizes dream state in humans. Proceedings of the National Academy of Sciences 90:2078–2081.

                                                                                                                                                                                                                                                                                                                                                                                                                DOI: 10.1073/pnas.90.5.2078Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                Demonstration of the possible involvement of gamma oscillations in producing dream cognition during REM sleep in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                • Stickgold, R., A. Malia, D. Maguire, D. Roddenberry, and M. O’Connor. 2000. Replaying the game: Hypnagogic images in normals and amnesics. Science 290.5490: 350–353.

                                                                                                                                                                                                                                                                                                                                                                                                                  DOI: 10.1126/science.290.5490.350Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                  First definitive demonstration of the replay of waking experience during imagery at sleep onset (hypnagogic imagery).

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                                                                                                                                                                                                                                                                                                                                                                                                                  • Voss, U., R. Holzmann, A. Hobson, et al. 2014. Induction of self awareness in dreams through frontal low current stimulation of gamma activity. Nature Neuroscience 17:810–812.

                                                                                                                                                                                                                                                                                                                                                                                                                    DOI: 10.1038/nn.3719Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                    This remarkable study showed that lucidity could be induced during sleep in lucid dreamers by applying the method of transcranial direct current stimulation to the frontal part of their brain.

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                                                                                                                                                                                                                                                                                                                                                                                                                    • Voss, U., R. Holzmann, I. Tuin, and J. A. Hobson. 2009. Lucid dreaming: A state of consciousness with features of both waking and non-lucid dreaming. Sleep 32.9: 1191–1200.

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                                                                                                                                                                                                                                                                                                                                                                                                                      Demonstration, using quantitative EEG, that lucid dreaming is associated with a level of activity of the frontal cortex intermediate between that of non-lucid REM sleep and waking.

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                                                                                                                                                                                                                                                                                                                                                                                                                      • Wamsley, E. J., M. Tucker, J. D. Payne, J. A. Benavides, and R. Stickgold. 2010. Dreaming of a learning task is associated with enhanced sleep-dependent memory consolidation. Current Biology 20.9: 850–855.

                                                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1016/j.cub.2010.03.027Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                        First demonstration of a correlation between dreaming of a task and improvement in its performance following waking.

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                                                                                                                                                                                                                                                                                                                                                                                                                        Demonstration of the Human PGO Wave

                                                                                                                                                                                                                                                                                                                                                                                                                        The PGO (pontine-geniculo-occipital) wave is an ascending (traveling from brainstem to cortex) potential (electrical activation), occurring during and just before REM sleep in the cat. The PGO wave travels from the pons in the brainstem to the lateral geniculate nucleus of the thalamus (LGN) to the occipital (visual) cortex. This wave was discovered with depth electrode recordings in the cat as well as in the rat (where it is termed the p-wave). The PGO wave was a prominent feature of the Activation-Synthesis Hypothesis of dreaming, being an example of a pseudo-sensory signal arriving at the cortex during sleep via a pathway dedicated to a sense organ (the visual pathway from LGN to occipital cortex) that, hence, could be interpreted by the brain as actual sensory input. For many years, experimental evidence for its existence in humans was sought using EEG and PET, but it was only with the added temporal resolution of MEG (Ioannides, et al. 2004), fMRI (Miyauchi, et al. 2009) and intracranial EEG (Lim, et al. 2007) that it has been conclusively demonstrated.

                                                                                                                                                                                                                                                                                                                                                                                                                        • Ioannides, A. A., M. Corsi-Cabrera, P. B. Fenwick, et al. 2004. MEG tomography of human cortex and brainstem activity in waking and REM sleep saccades. Cerebral Cortex 14.1: 56–72.

                                                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1093/cercor/bhg091Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                          Characterization of human brain activity during rapid eye movements in wake and REM sleep using MEG. Includes many figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                          • Lim, A. S., A. M. Lozano, E. Moro, et al. 2007. Characterization of REM-sleep associated ponto-geniculo-occipital waves in the human pons. Sleep 30.7: 823–827.

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                                                                                                                                                                                                                                                                                                                                                                                                                            Direct evidence of human PGO waves using depth electrodes placed in the pons of a patient with Parkinson’s disease.

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                                                                                                                                                                                                                                                                                                                                                                                                                            • Miyauchi, S., M. Misaki, S. Kan, T. Fukunaga, and T. Koike. 2009. Human brain activity time-locked to rapid eye movements during REM sleep. Experimental Brain Research 192.4: 657–667.

                                                                                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1007/s00221-008-1579-2Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                              Characterization of human brain activity associated with rapid eye movements using fMRI. Provides evidence for the existence of PGO waves in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                              Reviews of Sleep’s Role in Neuronal Plasticity

                                                                                                                                                                                                                                                                                                                                                                                                                              Conclusive demonstration of sleep-dependent memory consolidation in humans was preceded and accompanied by experimental evidence from animal research on ways in which the unique physiology of sleep could promote neuroplasticity—the structural changes and reorganization believed to underlie learning and memory. A prominent theory derived from work in animals (Buzsáki 1996), the hippocampo-neocortical dialogue, suggested that memories initially encoded in the hippocampus are transferred to more permanent storage in the cortex during sleep. Evidence for this theory came from demonstrations of hippocampal “replay” of neuronal firing sequences, that were recorded in prior waking, during NREM sleep (Pavlides and Winson 1989; Wilson and McNaughton 1994; Nadasdy, et al. 1999). Such replay was specifically linked to the hippocampal sharp wave and ripple complex (Siapas and Wilson 1998). REM sleep was also found to be associated with hippocampal replay (Louie and Wilson 2001). Numerous molecular events in the cell associated with memory consolidation have been proposed to occur during sleep (Graves, et al. 2001). This series of events leading to memory consolidation may depend on different stages of sleep occurring in a certain sequence (Giuditta, et al. 1995). REM sleep has also been hypothesized to promote adaptive forgetting of useless information (Crick and Mitchison 1983).

                                                                                                                                                                                                                                                                                                                                                                                                                              Major Reviews of Sleep-Dependent Memory Consolidation

                                                                                                                                                                                                                                                                                                                                                                                                                              The past two decades have seen an explosion of articles reporting on sleep’s enhancing effects on multiple human memory systems from declarative (memory for facts and events) to procedural memory (skills and habits) to evolutionarily primitive systems such as habituation, conditioning, and extinction. In the past decade, such studies have focused on the memory effects of specific sleep stages as well as their characteristic phasic events (such as stage-2 NREM sleep spindles). The following are excellent reviews bringing together these diverse findings to date (Diekelmann and Born 2010; Diekelmann, et al. 2009; Peigneux, et al. 2001; Stickgold 1998; Stickgold 2005; Walker and Stickgold 2006; Walker 2009). Recently, sleep-dependent consolidation has been strongly linked with the stage-2 NREM sleep spindle (Fogel and Smith 2011). The most recent comprehensive review is Rasch and Born 2013.

                                                                                                                                                                                                                                                                                                                                                                                                                              • Diekelmann, S., and J. Born. 2010. The memory function of sleep. Nature Reviews 11:114–126.

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                                                                                                                                                                                                                                                                                                                                                                                                                                Reviews the neurophysiological and neuromodulatory profile of NREM and REM sleep stages, behavioral evidence implicating these stages in memory processing, and conceptual models of sleep-dependent memory processing. Advances the view that SWS and REM sleep support different phases of memory consolidation.

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                                                                                                                                                                                                                                                                                                                                                                                                                                • Diekelmann, S., I. Wilhelm, and J. Born. 2009. The whats and whens of sleep-dependent memory consolidation. Sleep Medicine Reviews 13:309–321.

                                                                                                                                                                                                                                                                                                                                                                                                                                  DOI: 10.1016/j.smrv.2008.08.002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                  An overview of how sleep-dependent memory processing is influenced by several variables including characteristics of the learning material, learning mode, type of retrieval test, aspects of sleep, and the age and mental health of the subject population.

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                                                                                                                                                                                                                                                                                                                                                                                                                                  • Fogel, S. M., and C. T. Smith. 2011. The function of the sleep spindle: A physiological index of intelligence and a mechanism for sleep-dependent memory consolidation. Neuroscience & Behavioral Reviews 35:1154–1165.

                                                                                                                                                                                                                                                                                                                                                                                                                                    DOI: 10.1016/j.neubiorev.2010.12.003Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                    An overview of sleep spindles (bursts of 12–15 Hz EEG activity) and their relationship with IQ, learning, and memory. Includes figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                                    • Peigneux, P., S. Laureys, X. Delbeuck, and P. Maquet. 2001. Sleeping brain, learning brain. The role of sleep for memory systems. Neuroreport 12:A111–A124.

                                                                                                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1097/00001756-200112210-00001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                      Reviews multiple lines of evidence from both animal and human literature that sleep is important for memory consolidation.

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                                                                                                                                                                                                                                                                                                                                                                                                                                      • Rasch, B., and J. Born. 2013. About sleep’s role in memory. Physiological Reviews 93:681–766.

                                                                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1152/physrev.00032.2012Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                        The most recent and comprehensive review of sleep-dependent memory consolidation with over one thousand citations.

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                                                                                                                                                                                                                                                                                                                                                                                                                                        • Stickgold, R. 1998. Sleep: Off-line memory reprocessing. Trends in Cognitive Science 2:484–492.

                                                                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/S1364-6613(98)01258-3Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                          Early exposition on the theory of sleep-dependent memory consolidation.

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                                                                                                                                                                                                                                                                                                                                                                                                                                          • Stickgold, R. 2005. Sleep-dependent memory consolidation. Nature 437.7063: 1272–1278.

                                                                                                                                                                                                                                                                                                                                                                                                                                            DOI: 10.1038/nature04286Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                            Reviews the effect of sleep on various learning tasks including perceptual, procedural, and declarative types of memory. Focuses mainly on human studies. Includes figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                                            • Walker, M. P. 2009. The role of sleep in cognition and emotion. Annals of the New York Academy of Sciences 1156:168–197.

                                                                                                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1111/j.1749-6632.2009.04416.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                              Reviews literature regarding the effects of sleep on memory and creativity, as well as conceptual models of sleep-dependent memory processing. Additionally reviews the relationships among emotion, sleep, and memory. Proposes a model of sleep-dependent emotional memory processing. Includes many figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                                              • Walker, M. P., and R. Stickgold. 2006. Sleep, memory, and plasticity. Annual Review of Psychology 57:139–166.

                                                                                                                                                                                                                                                                                                                                                                                                                                                DOI: 10.1146/annurev.psych.56.091103.070307Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                Reviews the roles of sleep in memory encoding, consolidation, and reconsolidation (reprocessing of memories after initial consolidation), as well as how sleep changes memory-related brain activation. Includes figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                Early Primary Sources on Sleep-Dependent Memory Consolidation in Humans

                                                                                                                                                                                                                                                                                                                                                                                                                                                As in the case of several other topics above, it is highly instructive for the student to refer back to some of the original findings that spurred the growth of a field as diverse as sleep’s effects on learning and memory. A group of studies published in the late 1990s and early 2000s firmly established the existence of sleep-dependent memory consolidation and its association with brain changes. These reported improvement of a visuospatial skill associated with REM (Karni, et al. 1994), total sleep (Stickgold, et al. 2000a) or the sequence of sleep stages occurring across the night (Stickgold, et al. 2000b). Similarly, a procedural skill was associated with REM (Plihal and Born 1997) and stage-2 NREM (Walker, et al. 2002), and declarative memory was associated with slow wave sleep (Plihal and Born 1997; Plihal and Born 1999). These skills were, in turn, shown to be reflected in brain activity during sleep—analogously to the neural “replay” seen in animals (Maquet, et al. 2000; Laureys, et al. 2001).

                                                                                                                                                                                                                                                                                                                                                                                                                                                • Karni, A., D. Tanne, B. S. Rubenstein, J. J. M. Askenasy, and D. Sagi. 1994. Dependence on REM sleep of overnight improvement of a perceptual skill. Science 265:679–682.

                                                                                                                                                                                                                                                                                                                                                                                                                                                  DOI: 10.1126/science.8036518Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                  Early demonstration that consolidation of perceptual learning occurs during REM sleep in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                  • Laureys, S., P. Peigneux, C. Phillips, et al. 2001. Experience-dependent changes in cerebral functional connectivity during human rapid eye movement sleep. Neuroscience 105:521–525.

                                                                                                                                                                                                                                                                                                                                                                                                                                                    DOI: 10.1016/S0306-4522(01)00269-XSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                    Characterization, using PET, of learning-dependent changes in functional connectivity in learning-related brain areas during REM sleep.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                    • Maquet, P., S. Laureys, P. Peigneux, et al. 2000. Experience-dependent changes in cerebral activation during human REM sleep. Nature Neuroscience 3:831–836.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                      Demonstration, using PET, that brain areas active during a visuomotor task are reactivated during subsequent REM sleep in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                      • Plihal, W., and J. Born. 1997. Effects of early and late nocturnal sleep on declarative and procedural memory. Journal of Cognitive Neuroscience 9.4: 534–547.

                                                                                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1162/jocn.1997.9.4.534Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                        First direct evidence in humans that different sleep stages benefit different types of memories.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                        • Plihal, W., and J. Born. 1999. Effects of early and late nocturnal sleep on priming and spatial memory. Psychophysiology 36:571–582.

                                                                                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1111/1469-8986.3650571Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                          Evidence that different stages of sleep are involved in the consolidation of declarative and non-declarative memories in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                          • Stickgold, R., L. James, and J. A. Hobson. 2000a. Visual discrimination learning requires sleep after training. Nature Neuroscience 3:1237–1238.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                            Evidence that perceptual memory consolidation depends on the first night of sleep following learning.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                            • Stickgold, R., D. Whidbee, B. Schirmer, V. Patel, and J. A. Hobson. 2000b. Visual discrimination task improvement: A multi-step process occurring during sleep. Journal of Cognitive Neuroscience 12:246–254.

                                                                                                                                                                                                                                                                                                                                                                                                                                                              DOI: 10.1162/089892900562075Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                              Evidence that both slow wave and REM sleep stages are involved in consolidation of perceptual learning in humans.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                              • Walker, M. P., T. Brakefield, A. Morgan, J. A. Hobson, and R. Stickgold. 2002. Practice with sleep makes perfect: Sleep-dependent motor skill learning. Neuron 35:205–211.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                DOI: 10.1016/S0896-6273(02)00746-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                                Demonstration that sleep, particularly stage-2 NREM sleep, is important for consolidation of motor sequence learning.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                Sleep and Developmental Plasticity

                                                                                                                                                                                                                                                                                                                                                                                                                                                                The abundance of REM sleep in neonatal mammals, including humans, has led to much speculation that this sleep stage is involved in early neuronal development. The following studies provide experimental evidence from animal models showing that this is indeed likely. For example, sleep has been shown necessary for optical dominance plasticity in the cat (Frank, et al. 2001), with mechanistic explanations being continuously added with further study as reviewed in Aton, et al. 2009 and Frank 2011. It has long been speculated that the great abundance of REM in early infancy is involved in brain development (Marks, et al. 1995; Scher 2007).

                                                                                                                                                                                                                                                                                                                                                                                                                                                                Other Theories on Functional Roles for Sleep

                                                                                                                                                                                                                                                                                                                                                                                                                                                                Although memory consolidation has received much recent attention as a likely function of sleep, it is undoubtedly only one of a number of sleep’s adaptive functions. Indeed, given that sleep-like states evolved in primitive organisms, it is more likely that the original function of sleep involved more basic metabolic or ecological needs. Sleep as an ancestral behavior, like other biological structures and behaviors, has undoubtedly been modified by natural selection to perpetuate individual species in specific environments.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                Reviews on Theories of Sleep Function

                                                                                                                                                                                                                                                                                                                                                                                                                                                                The following early reviews (Hobson 1989; Rechtschaffen 1998) nicely summarize theories on the function of sleep extant at the time of their publication. Notably, this issue has not been resolved to this day although increasingly sophisticated theories are appearing.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                • Hobson, J. A. 1989. Sleep. New York: Scientific American Library.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                  Cited above as a general reference on sleep, this volume also provides an excellent discussion on early theories on the function of sleep.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                  • Rechtschaffen, A. 1998. Current perspectives on the function of sleep. Perspectives in Biological Medicine 41:359–390.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Reviews experimental studies of sleep and theories of sleep function. Argues that sleep serves an as-yet undiscovered fundamental and universal function.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Ecological Optimization

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    A prominent theory drawn from the study of sleep across many different phyla states that sleep behaviors serve ecological functions that enhance species’ survival and reproductive fitness. Jerome Siegel (Siegel 2008, Siegel 2009) has argued extensively in favor of this topic drawing from data across the phyla from invertebrates to poikilothermic vertebrates to homeotherms and especially those having sleep laterality adapted to their ecological niche (e.g., marine mammals).

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Metabolic Homeostasis

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Benington and Heller 1995 is one of a number of theories that suggest specific restorative physiological roles for sleep.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Cellular Homeostasis and Metabolism

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Perhaps getting closer to a putative primitive function of sleep are findings that link sleep to intracellular housekeeping functions. Increasingly, roles of sleep are being sought at the cellular and molecular level including optimization of cellular metabolic (Mackiewicz, et al. 2007), cellular “housekeeping” such as disposal of misfolded proteins (Naidoo 2009), maintenance of an appropriate balance and prioritization of synaptic potentiation and depotentiation (Tononi and Cirelli 2006), and even mechanical disposal of metabolic waste via interstitial fluid (Xie, et al. 2013).

                                                                                                                                                                                                                                                                                                                                                                                                                                                                    • Mackiewicz, M., K. R. Shockley, M. A. Romer, et al. 2007. Macromolecule biosynthesis: A key function of sleep. Physiological Genomics 31:441–457.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                      DOI: 10.1152/physiolgenomics.00275.2006Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Characterization of gene expression in the mouse cortex and hypothalamus during sleep.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                      • Naidoo, N. 2009. Cellular stress/the unfolded protein response: Relevance to sleep and sleep disorders. Sleep Medicine Reviews 13:195–204.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                        DOI: 10.1016/j.smrv.2009.01.001Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Reviews the molecular pathways involved in the ER (endoplasmic reticulum) stress response and how this response interacts with sleep and aging. Includes helpful figures.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                        • Tononi, G., and C. Cirelli. 2006. Sleep function and synaptic homeostasis. Sleep Medicine Reviews 10:49–62.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                          DOI: 10.1016/j.smrv.2005.05.002Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                                          Although specifically pertaining to higher organisms and to sleep-related effects on learning and memory, the “synaptic homeostasis hypothesis” of sleep function also provides a function for sleep that operates at the cellular and subcellular level and, hence, might have arisen early in the evolution of this behavior.

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                                                                                                                                                                                                                                                                                                                                                                                                                                                                          • Xie, L., H. Kang, Q. Xu, et al. 2013. Sleep drives metabolite clearance from the adult brain. Science 342:373–377.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                            DOI: 10.1126/science.1241224Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                                                                                                                                                                                                                                                                                            A recent study showing that sleep accelerates waste metabolite clearance from the brain.

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