<P> During slow - wave sleep, humans secrete bursts of growth hormone . All sleep, even during the day, is associated with secretion of prolactin . </P> <P> Key physiological measurements indicators of sleep include EEG of brain waves, electrooculography (EOG) of eye movements, electromyography (EMG) of skeletal muscle activity . Simultaneous collection of these measurements is called polysomnography, and can be performed in a specialized sleep laboratory . Sleep researchers also use simplified electrocardiography (EKG) for cardiac activity and actigraphy for motor movements . </P> <P> Sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM sleep) and rapid eye movement (REM sleep). non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states . Non-REM sleep occurs first and after a transitional period is called slow wave sleep or deep sleep . During this phase, body temperature and heart rate fall, and the brain uses less energy . REM sleep (also known as paradoxical sleep), a smaller portion of total sleep time and the main occasion for dreams (or nightmares), is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone, and suspension of homeostasis . </P> <P> The sleep cycle of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4--6 times in a good night's sleep . The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow - wave sleep . The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM . REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep . There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening . </P>

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