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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.
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]
McCarley and Hobson suggested that the REM-on neurons actually stimulate REM-off neurons, thereby serving as the mechanism for the cycling between REM and non-REM sleep. [3] [22] [24] [28] They used Lotka–Volterra equations to describe this cyclical inverse relationship.
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]
The rem and millirem are CGS units in widest use among the U.S. public, industry, and government. [4] However, the SI unit the sievert (Sv) is the normal unit outside the United States, and is increasingly encountered within the US in academic, scientific, and engineering environments, and have now virtually replaced the rem. [5]
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.
In addition, nightmares appear ordinarily during REM sleep in contrast to night terrors, which occur in NREM sleep. [2] Finally, individuals with nightmares can wake up completely and easily and have clear and detailed memories of their dreams. [2] [30] A distinction between night terrors and epileptic seizure is required. [30]