REM sleep

You might expect that the person would stay in Stage 4 (deep sleep) for five or six hours, then gradually move into lighter sleep as morning approaches. However, a person typically remains in Stage 4 only a short time before moving back through the stages in reverse order. After the person passes through Stage 2, the EEG tracings become fast and irregular, similar to the pattern of alert wakefulness.



But the person is still sound asleep, in a new sleep stage marked by almost complete muscle relaxation except for the eye muscles, which are very active. This stage of sleep is called rapid eye movement sleep, or REM sleep.