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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.
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. [18] 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. [15]
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.
The sleep cycle has proven resistant to systematic alteration by drugs. Although some drugs shorten REM periods, they do not abolish the underlying rhythm. Deliberate REM deprivation shortens the cycle temporarily, as the brain moves into REM sleep more readily (the "REM rebound") in an apparent correction for the deprivation. [13]
Emotional selection is a psychological theory of dreaming that describes non-REM dreams as modifying mental schemas and REM dreams as testing prior non-REM modifications. [1] [2] Schemas hold concepts that contribute to a mental model of the self and external reality. Consequently, according to emotional selection theory, dreams modify and test ...
Sleep deprivation increases this propensity, which can be measured by polysomnography (PSG) as a reduction in sleep latency (the time needed to fall asleep). [69] An indicator of sleep propensity can also be seen in the shortening of the transition from light stages of non-REM sleep to deeper slow-wave oscillations. [69]
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 ]
[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.