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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]
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 ...
[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.
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 ]
[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]
NREM Stage 1 (N1 – light sleep, somnolence, drowsy sleep – 5–10% of total sleep in adults): This is a stage of sleep that usually occurs between sleep and wakefulness, and sometimes occurs between periods of deeper sleep and periods of REM. The muscles are active, and the eyes roll slowly, opening and closing moderately.