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  • Sleep Consists of Alternating REM and Non-REM Periods

    • Non-REM Sleep Has Four Stages

    • REM and Non-REM Dreams Are Different

  • Sleep Obeys Circadian and Ultradian Rhythms

    • The Circadian Rhythm Clock Is Based on a Cyclic Production of Nuclear Transcription Factors

    • The Ultradian Rhythm of Sleep Is Controlled by the Brain Stem

    • Sleep-Related Activity in the EEG Is Generated Through Local and Long-Range Circuits

  • Sleep Changes with Age

  • The Characteristics of Sleep Vary Greatly Between Species

  • Sleep Disorders Have Behavioral, Psychological, and Neurological Causes

    • Insomnia Is the Most Common Form of Sleep Disruption

    • Excessive Daytime Sleepiness Is Indicative of Disrupted Sleep

    • The Disruption of Breathing During Sleep Apnea Results in Fragmentation of Sleep

    • Narcolepsy Is Characterized by Abnormal Activation of Sleep Mechanisms

    • Restless Leg Syndrome and Periodic Leg Movements Disrupt Sleep

    • Parasomnias Include Sleep Walking, Sleep Talking, and Night Terrors

    • Circadian Rhythm Sleep Disorders Are Characterized by an Activity Cycle That Is Out of Phase with the World

  • An Overall View

Sleep is a remarkable state. It consumes fully a third of our lives—approximately 25 years in the average lifetime—yet most of us know little about this daily excursion into our inner world. Perhaps even more surprising, we are still hard pressed to give a raison d'etre for sleep.

The exact functions of sleep and of dreaming, one of the more spectacular components of sleep, have been debated over the ages and are still not known. Do dreams reveal some inner psychological functioning of unconscious mental processes, as Sigmund Freud first suggested, or are they merely the consequence of random firing of neurons in the brain? The psychic content of dreams has been a rich subject of speculation throughout history. Plato, anticipating Sigmund Freud, thought that all of us have a "terrible, fierce, and lawless blood of desires, which it seems are revealed in our sleep," whereas Aristotle believed that dreams are merely afterthoughts of the day's activities and experiences.

Although we have only limited understanding of the functions of sleep, our insight into its mechanisms has increased greatly over the past 50 years. We have moved away from the intuitively appealing but incorrect notion that sleep is a period of relative inactivity and rest—one that occurs reflexively in response to reduction of sensory input—to the current view that sleep is a highly organized state generated by the cooperative interplay of many behavioral and neural components. Although in many respects the biological function of sleep remains a mystery, we have begun to understand the cellular and molecular processes underlying sleep.

Sleep Consists of Alternating REM and Non-REM Periods

Sleep affects all of our bodily and mental functions, from the regulation of hormonal levels to muscle tone, from the regulation of respiration rate to the content of our thought processes. Given these important behavioral changes, it is not surprising that the brain's overall electrical ...

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