Medical Terminology

sleep stages

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Sleep is made up of four stages – stages 1, 2, 3 and REM (rapid eye movement). Stages 1, 2 and 3 are generally non-rapid-eye-movement (NREM), which typically makes up about 75% of your time asleep, while the REM stage makes up the other 25% of sleep. As you progress from wakefulness to deep sleep, you usually pass through progressively deeper stages, from stage 1 (the lightest) to stage 3, or slow wave sleep (the deepest stage, in which critical substances, such as human growth hormone, are released), and finally into REM sleep (during which the body doesn’t move and dreaming occurs). It is not uncommon for a person to move back and forth between stages 2 and 3 before moving into REM sleep. A typical 4-stage sleep cycle lasts about 90-110 minutes, and then the sleep cycle repeats with Stage 1. More stage 3 sleep occurs earlier in the sleep period, while more REM sleep occurs later in the sleep period. For example, the first period of REM sleep generally lasts about 10 minutes, with each later REM stage getting longer – and the final REM stage may last up to an hour.