n. a biological cycle which alternates between periods of wakefulness and sleep - or in other words, activity and non-activity. Typically, it goes in rhythm with your human biological clock, and it cycles every 80 to 120 minutes (90 minutes on the average).
BASIC REST-ACTIVITY CYCLE (BRAC): "The Basic Rest-Activity Cycle (BRAC) is a transition from different levels of excitable to restful states and then back."
