Basic rest activity cycle

The existence of a Basic Rest - Activity Cycle, or BRAC, during both sleep and wakefulness was proposed by Nathaniel Kleitman.

It is a human biological cycle of approximately 90 minutes (80–120 minutes) that is characterized by different level of excitement and rest. The cycle is controlled by the human biological clock. It is best observed in stages of sleep, for example, REM and the delta activity cycle.