"And you cycle throughout so that you do about five to six cycles throughout the night. And we spend more time in REM later on in the night than we do earlier on."

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So we go through in the beginning of the night, we go into the really deep stages of sleep and we actually cycle through. So, when you go down to the deep stage, then you go back up and you actually come into something called REM sleep, which is after about 90 minutes.

— Shelby Harris