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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.
[1] [2] Unlike before, where wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep were considered exclusive states, research has shown that combinations of these states are possible and thus, may result in unusual unstable states that could eventually manifest as parasomnias or as altered levels of awareness.
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).
[49]: 689 Slow-wave sleep, part of non-REM sleep, appears to be important for declarative memory. Artificial enhancement of the non-REM sleep improves the next-day recall of memorized pairs of words. [74] Tucker et al. demonstrated that a daytime nap containing solely non-REM sleep enhances declarative memory—but not procedural memory. [75]
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] In humans, the transition between non-REM and REM is abrupt; in other animals, it is less so. [11]
According to the Hobson & McCarley activation-synthesis hypothesis, proposed in 1975–1977, the alternation between REM and non-REM can be explained in terms of cycling, reciprocally influential neurotransmitter systems. [13] Sleep timing is controlled by the circadian clock, and in humans, to some extent by willed behavior.
Night terror, also called sleep terror, is a sleep disorder causing feelings of panic or dread and typically occurring during the first hours of stage 3–4 non-rapid eye movement (NREM) sleep [1] and lasting for 1 to 10 minutes. [2] It can last longer, especially in children. [2]