At the broadest level, our activity and rest patterns are tied to circadian rhythms (circa dies, “around a day”), which cycle approximately every twenty-four hours. In the early 1950s, researchers Eugene Aserinsky and Nathan Kleitman discovered that sleep occurs in smaller cycles of 90-to 120-minute segments. We move from light sleep, when brain activity is intense and dreaming occurs, to deeper sleep, when the brain is more quiescent and the deepest restoration takes place. This rhythm is called the “basic rest-activity cycle” (BRAC). In the 1970s, further research showed that a version of
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