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Non-rapid eye movement sleep is known for its beneficial effect on memory consolidation, especially for declarative memory (while procedural memory improvement is more associated with REM-sleep), [26] even if establishing a clear-cut distinction between stages' influence on type of learning may not be possible.
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.
In the weeks after a human baby is born, as its nervous system matures, neural patterns in sleep begin to show a rhythm of REM and non-REM sleep. (In faster-developing mammals, this process occurs in utero.) [57] Infants spend more time in REM sleep than adults. The proportion of REM sleep then decreases significantly in childhood.
Rapid eye movement sleep (REM), non-rapid eye movement sleep (NREM or non-REM), and waking represent the three major modes of consciousness, neural activity, and physiological regulation. [8] NREM sleep itself is divided into multiple stages – N1, N2 and N3.
Homeostatic functions, especially thermoregulation, occur normally during non-REM sleep, but not during REM sleep. Thus, during REM sleep, body temperature tends to drift away from its mean level, and during non-REM sleep, to return to normal. Alternation between the stages therefore maintains body temperature within an acceptable range. [10]
Because REM sleep is detectable in many species, and because research suggests that all mammals experience REM, [15] linking dreams to REM sleep has led to conjectures that animals dream. However, humans dream during non-REM sleep, also, and not all REM awakenings elicit dream reports. [16]
Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves. [ 2 ] Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night. [ 3 ]
Sleep onset is the transition from wakefulness into sleep. Sleep onset usually transits into non-rapid eye movement sleep (NREM sleep) but under certain circumstances (e.g. narcolepsy ) it is possible to transit from wakefulness directly into rapid eye movement sleep (REM sleep).