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 the same 90-to 120-minute cycles—ultradian rhythms (ultra dies, “many times a day”)—operates in our waking lives.
