Human Sleep: Stages, Timing, and Disorders

Definition and core features of sleep

Sleep is a naturally recurring state of reduced mental and physical activity in which consciousness is altered and sensory responsiveness is inhibited. Compared with wakefulness, sleepers react less to stimuli, yet sleep still involves active brain patterns, making it more responsive than a coma or disorders of consciousness. During sleep, muscle activity decreases markedly and interaction with the environment is reduced. A core feature of sleep is that it occurs in repeating periods in which the body alternates between two distinct modes: non-REM (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep typically begins first and includes slow-wave (deep) sleep, when body temperature and heart rate fall and the brain uses less energy. REM sleep, though it has rapid eye movements, is also marked by desynchronized and fast brain waves, loss of muscle tone (near paralysis), and is strongly associated with dreaming. Sleep also supports bodily restoration: during sleep many systems shift toward an anabolic state that helps restore the immune, nervous, skeletal, and muscular systems, supporting mood, memory, and cognitive function. The timing of sleep is regulated by an internal circadian clock (promoting sleep at night) and sleep-wake homeostasis (sleep pressure that increases with time awake). Sleep can be monitored using EEG (brain waves), EOG (eye movements), and EMG (muscle activity), often combined as polysomnography.

0/2 modes complete

REM and non-REM as alternating sleep modes

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