Ever pulled an all-nighter, then started to feel alert around 7 or 8 a.m.? That was your internal clock jolting you awake just when you wished you could crawl under the covers.
Researchers know that when and how long our bodies want to sleep are separate but interrelated behaviors. One system, the internal clock, controls the daily, or circadian, rhythm of when we have the urge to go to bed and wake up.
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