The sleep cycle is divided into two main stages: non-rapid eye movement (non-REM) and rapid eye movement (REM).
Non–rapid eye movement sleep has three stages:
Stage N1 occurs right after you fall asleep and is very short (usually less than 10 minutes). It involves light sleep from which you can be awakened easily.
Stage N2 lasts from about 30 to 60 minutes. During this stage, your muscles become more relaxed and you may begin to have slow-wave (delta) brain activity.
Stage N3 is deep sleep and lasts about 20 to 40 minutes. During this stage, delta brain activity increases and you may have some body movements. It is very hard to wake up someone in stage N3.
Most dreaming happens during REM sleep. During REM sleep:
The eyes and eyelids flutter.
Breathing becomes irregular. During REM sleep, it is normal to have short episodes when breathing stops (apnea).
The brain paralyzes your muscles so you do not act out the dreams.
During sleep, a person usually progresses through the 3 stages of non-REM sleep before entering REM sleep. This takes about 1 to 2 hours after falling asleep. The sleep cycle is usually repeated about 4 to 6 times each night.
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