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Sleep is a naturally recurring state of reduced mental and physical activity in which consciousness is altered and sensory processing is inhibited. Compared with wakefulness, sleepers react less to stimuli, yet sleep is not “unconscious” in the same way as a coma; it involves active brain patterns that keep the person more responsive than disorders of consciousness. During sleep, muscle activity decreases markedly and interaction with the environment is reduced. A key basic feature of sleep is that it occurs in repeating periods in which the body alternates between two distinct modes: non-REM (NREM) and rapid eye movement (REM) sleep. NREM sleep typically occurs first and includes slow-wave (deep) sleep, when heart rate and body temperature fall and the brain uses less energy. REM sleep is a smaller portion of total sleep time, is strongly associated with dreaming, and features desynchronized/fast brain activity, rapid eye movements, loss of muscle tone, and changes in homeostasis. The full NREM–REM cycle averages about 90 minutes and repeats several times during a typical night.
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