Sleep is an active process, resulting from the interactions of neuronal networks within the brain. These systems have some redundancy such that lesions of the central nervous system temporarily interrupt the ability to access sleep. This reduplication of systems involved in sleep activation and maintenance demonstrates the necessity of sleep, even though we do not understand the full purpose of sleep.
Sleep is divided into stages on the basis of electroencephalography (EEG), eye movements, and muscle tone. The primary division is between rapid eye movement (REM) sleep and non–rapid eye movement (NREM) sleep:
NREM sleep has four stages:
REM sleep is characterized by a low-amplitude mixed frequency pattern on EEG, absence of muscle tone in voluntary muscles, and intermittent rapid eye movements.
Sleep is a maturational process. Sleep time is greatest in infancy, divided equally between REM and NREM sleep.
By the end of the first decade of life, sleep has developed its basic adult pattern. Healthy adults display a reproducible pattern of sleep organization. Normal sleep architecture is composed of progressive cycles in which the majority of delta or slow-wave (stages 3 and 4) sleep occurs in the first one-third of the sleep period and the majority of REM sleep in the later one-third. At the onset of sleep, the individual displays stage 1 sleep, progresses to stage 2, and subsequently enters stage 3 and stage 4. After approximately 80–90 minutes, the first REM episode occurs, lasting approximately 5–10 minutes. The individual generally returns to stage 1 or 2 sleep before entering delta sleep. The next episodes of REM occur in approximately 60- to 90-minute cycles and gradually lengthen through the night. For most adults, approximately 50% of the sleep period is spent in stage 2 sleep, approximately 20% in stages 3 and 4, and approximately 25% in REM sleep. Adults may normally awaken 4 to 6 times and have several shifts to lighter stages of sleep during the night.
With advancing age, the amount of stage 4 (and some stage 3) sleep decreases, and sleep fragmentation increases, owing to spontaneous arousals.
For more detailed information regarding sleep disorders and the neurobiology of sleep, the reader is referred to several comprehensive sleep medicine texts.23–25
Reviewed and revised April 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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