Sleep physiology, timing, functions, and disorders

Definition and core characteristics 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 is not “inactive”—it involves active brain patterns and is more organized than states such as coma or disorders of consciousness. During sleep, muscle activity decreases and interactions with the surrounding environment are reduced. A core characteristic of sleep is that it occurs in repeating cycles in which the body alternates between two distinct modes: non-REM (NREM) sleep 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, which makes up a smaller portion of total sleep time, is strongly associated with dreaming and features desynchronized, fast brain activity, eye movements, reduced muscle tone, and changes in physiological regulation. Sleep is also regulated by internal timing systems and measurable physiological changes. The circadian clock promotes sleep at night, largely through light-driven hormonal signaling (including melatonin), while sleep-wake homeostasis (“sleep debt”) increases the drive to sleep the longer one stays awake. Researchers monitor sleep using tools such as EEG (brain waves), EOG (eye movements), and EMG (muscle activity), often combined as polysomnography.

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Alternating REM and non-REM sleep modes

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