Normal sleep progressively passes through five stages. The stages of sleep are defined by their associated electrical patterns in the brain. While a person sleeps, it is possible to detect these patterns with a device which senses the electrical activity in the brain, called an electroencephalogram (EEG).
During the first four stages of sleep, the muscles of the eyes are relaxing. These stages are collectively referred to as nonrapid eye movement (nonREM) sleep.
The last stage of sleep is associated with increased contraction of the eye muscles. As a result, the fifth stage of sleep (when eye movement is active) is called rapid eye movement sleep or REM sleep.
Sleep requirements vary from person to person. Some individuals may be refreshed upon awakening after only four to five hours of sleep, while others need far more than the average (eight hours). The feeling of inadequate sleep despite an adequate opportunity to sleep is referred to as insomnia.
Quick GuideSleep Disorders: Insomnia, Sleep Apnea, and More
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