The average night's sleep of 7-8 hours, cycles through different stages based on two broad categori...
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The average night's sleep of 7-8 hours, cycles through different stages based on two broad categories, called REM, for rapid eye-movement, and non-REM sleep. Most dreams occur during REM sleep; there are typically four or five periods that get progressively longer during a night's sleep. Non-REM sleep is divided into stages. For many years, there were 4 stages, but several years ago, 3 and 4 - the deepest stages of sleep - were merged into a single stage 3. Throughout the night, a person cycles through light to deep stages of non-REM sleep, interspersed by cycles of REM sleep.

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