Basic rest–activity cycle

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.

Kleitman hypothesized that the short-term 50 minute ultradian cycle of infants observed by researchers Denisova and Figurin ensured that a newborn infant would have frequent opportunities to respond to the stimulus of hunger pangs by waking up and crying, and would therefore get adequate nutrition. This hypothesis influenced him to consider BRAC a fundamental human physiological process.[3]

Kleitman concluded that BRAC tended to lengthen somewhat with age, based on research published from Ohlmeyer and Brilmeyer.[4]