Stages of sleep

Sleep stages are divided into non–rapid eye movement (non-REM) and rapid eye movement (REM).

Non-REM sleep

Non–rapid eye movement (non-REM) sleep has four stages:

• Stages 1 and 2 are light sleep in which breathing is slower than when a person is awake.
• Stages 3 and 4 are called slow-wave (delta) sleep, in which the person's rate of breathing slows down further.

REM 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).

During sleep, a person usually progresses through the four stages of non-REM sleep before entering REM sleep. This takes about 60 to 90 minutes after falling asleep. The cycle is repeated three to four times each night with more time spent in the REM sleep stage and less time in sleep stages 3 and 4.