Basic rest–activity cycle

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The basic rest–activity cycle (BRAC) is a physiological arousal mechanism in humans proposed by Nathaniel Kleitman,[1] hypothesized to occur during both sleep and wakefulness.

Empirically, it is an ultradian rhythm of approximately 90 minutes (80–120 minutes[2]) that is characterized by different level of excitement and rest. The cycle is mediated by the human biological clock. It is most readily observed in stages of sleep, for example, rapid eye movement sleep (REM) and the delta activity cycle.

When awake, our brainwaves are faster during the first half of the cycle, when we feel alert and focused, and then our brainwaves slow; in the last 20 minutes when we feel dreamy and perhaps a little tired, while our body is being readied for the alert part of the following cycle.

When asleep, our brainwaves first slow and then rise, and the BRAC occurs as stages of sleep—first falling into deep sleep, then rising into the REM stage, when dreams occur.[3]


  1. ^ Kleitman, N., Sleep and Wakefulness, 1963, Reprint 1987: ISBN 9780226440736
  2. ^ Kleitman, N., Basic rest-activity cycle—22 years later, Journal of Sleep Research & Sleep Medicine, Vol 5(4), Dec 1982, 311–317
  3. ^ "Basic Rest and Activity Cycles," Polyphasic Society