Sleep cycle

The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes.

Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep.
These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0

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