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After a few minutes of stage 1 sleep, people move into stage 2 sleep. Stage 2 lasts about twenty minutes and is characterized by short bursts of brain waves called sleep spindles. People then pass into slow-wave sleep, which occurs during stages 3 and 4. In stages 3 and 4, which together last about thirty minutes, the EEG displays mostly delta waves. People in stage 3 and 4 sleep show slow breathing and pulse rates, have limp muscles, and are difficult to rouse.

REM Sleep

At the end of stage 4, people go back through the stages in reverse, from stage 4 to 3 to 2 to 1. When they reach stage 1, instead of waking up, people go into REM, or rapid eye movement, sleep. A single cycle might look like this:

1 2 3 4 3 2 REM

REM sleep is a stage of deep sleep in which, paradoxically, brain wave activity resembles that of an alert person. REM sleep is also called paradoxical sleep.

During REM sleep, pulse rate and breathing become irregular, eyes move rapidly under closed lids, and muscles remain very relaxed. Genital arousal also happens during REM. In women, the clitoris becomes swollen with blood, and vaginal lubrication increases. In men, the penis becomes erect. EEGs show mostly beta waves during REM sleep. Although dreaming happens in other sleep stages as well, dreams are most vivid and frequent during REM sleep.

People typically go through about four sleep cycles during one night of sleep. The REM stage of sleep gets longer and longer as the night passes, while stage 3 and 4 sleep gets shorter and shorter. During the night’s first sleep cycle, the REM stage lasts about ten minutes. During the night’s last sleep cycle, people may spend about forty to sixty minutes in REM sleep. Non-REM sleep becomes more shallow as the night goes on, and eventually the sleeper awakens.

Sleep Deprivation

Different people need different amounts of sleep. Some people can function with fewer than six hours of sleep a night, while others can’t manage without at least nine hours. Research shows that getting insufficient sleep can have negative effects on health, productivity, and performance.

Researchers have also studied the effects of insufficient REM sleep. Experiment subjects who are intentionally deprived of REM sleep tend to enter the REM stage of sleep more and more frequently during the night. After an REM-deprivation experiment has ended, subjects usually experience a REM rebound effect, spending more time in the REM stage on subsequent nights to make up for lost REM time.

Aging and Sleep

Sleep patterns change as people get older. Newborn babies spend about two-thirds of their time in sleep. As people age, they tend to sleep less. The amount of time spent in REM sleep also changes over time. In very young babies, about half of all sleep is REM sleep. As babies get older, the proportion of REM sleep decreases.

Sleep Disorders

Everyone has occasional difficulty sleeping, but some people have insomnia, a chronic problem with falling or staying asleep. Another kind of sleep disorder is narcolepsy, which is a tendency to fall asleep periodically during the day. Narcolepsy can be dangerous, as people who experience it may fall asleep while driving or operating machinery.

Sleep apnea is another condition that can have negative effects on health and safety. People who have sleep apnea stop breathing many times during a night’s sleep, and each time they stop breathing, they wake up briefly and gasp for air. This prevents them from getting enough deep sleep, which leads to irritability and sleepiness during the day. Chronic sleep apnea can also result in high blood pressure.