Sleep itself is not a unified state but consists of five distinct stages: one stage of rapid eye movement (REM) sleep and four stages of nonrapid eye movement (NREM) sleep. During a typical night's sleep, one moves in and out of these stages four or five times. REM sleep is primarily associated with periods of dreaming. Sleeping subjects awakened during a period of REM sleep report having just experienced a dream about 80 percent of the time, compared with less than 10 percent when NREM sleep is interrupted. Psychologists are still unclear on exactly why humans need to sleep, but the need for periods of REM sleep might be part of the reason. When sleeping subjects are deprived of REM sleep (and their NREM sleep is undisturbed), they often show many of the symptoms of not having slept at all. Also, when later given the opportunity for uninterrupted sleep, they spend a greater percentage of time in the REM stage, as if making up for the lost REM sleep (this is referred to as the REM-rebound effect). The REM-rebound effect is lessened if the individual is encouraged to engage in an increased amount of daydreaming, which indicates a possible connection between day and night dreams.
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