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Stages of Sleep

Stage 1:
Light sleep. We drift in and out and can be awakened easily. Our eyes move slowly and muscle activity slows.

Stage 2:
Our eye movements stop and our brain waves become slower with occasional bursts of rapid waves called sleep spindles.

Stage 3:
Deep sleep. Extremely slow brain waves called delta waves appear, interspersed with smaller, faster waves.

Stage 4:
Deep sleep. The brain produces mostly delta waves. There are no eye movements and no muscle activity.

Stage 5:
REM sleep. Breathing becomes more rapid, irregular, and shallow. Eyes jerk rapidly, limb muscles become temporarily paralyzed. Dreams almost always happen in this stage, but may occur in other sleep stages as well.

It takes about 2 hours to go through all five stages of sleep, and then they repeat. Rapid eye movement (REM) sleep usually occurs about 90 minutes after we fall asleep. Adults spend half of their sleep time in stage 2, 20 percent of the time in REM sleep, and 30 percent in the other stages. Infants start out spending about half of their sleep time in REM sleep. Older people spend relatively little time in stages 3 and 4.

(Source: National Institutes of Health)