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.
When awake, our brainwaves are faster during the first half of our BRAC, when we feel alert and focused, and then our brainwaves slow during the second half, till we reach the last 20 minutes when we feel dreamy and perhaps a little tired, while our body’s being readied for the alert part of the following BRAC; 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 (Rapid Eye-Movement) stage, when dreams occur.