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. 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. Stage 3 is the deepest stage, in which critical substances, such as human growth hormone, are released. After that, you progress 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 session, while more REM sleep occurs later in the sleep session. For example, the first occurrence of REM sleep in a sleep session generally lasts about 10 minutes. Each later REM stage gets longer, with the final REM stage lasting up to an hour.


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