Sleep stages are divided into non–rapid eye movement (non-REM) and rapid eye movement (REM).
Non-REM (NREM) sleep
Non–rapid eye movement (NREM) sleep has 3 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 a person may have some body movements. It is very hard to wake up someone in stage N3.
REM (R) sleep
Rapid eye movement sleep is deeper than non-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).
You do most of your dreaming during REM sleep. But your 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 cycle is repeated three to four times each night. An adult spends more time in NREM sleep than in REM sleep. An infant usually spends about half of the sleeping time in NREM and half in REM stages.
Medical Review:Anne C. Poinier, MD - Internal Medicine & Adam Husney, MD - Family Medicine & Hasmeena Kathuria, MD - Pulmonology, Critical Care Medicine, Sleep Medicine