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sleep stage

(sleep stayj)
One of four stages of the sleep cycle based on the type of brain activity that occurs during the stage. Stages one, two, and three occur during non-rapid eye movement (non-REM) sleep, which is when a person first falls asleep and then moves from a light sleep into a deep sleep. Stage four occurs during REM sleep, which is when the eyes move rapidly while closed and when most dreams occur. During normal sleep, a person goes through four to five sleep cycles that last about 90 minutes each and include both non-REM sleep and REM sleep.
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