Sleep stages are dividedinto 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 inwhich 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 downfurther.

REM sleep

Rapid eye movement sleep isdeeper 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 breathingstops (apnea).

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