1. Supposed regular cycles in human physiological processes that affect emotions and behavior.
2. A biologically inherent cyclic variation or recurrence of an event or state; such as, the sleep cycle, circadian rhythms, or periodic diseases.
3. An endogenous cyclic variation in some aspect of an organism's bodily functioning, as the daily cycle of sleeping and waking, or the annual cycle of dormancy and activity in some animals; specifically each of three alleged cycles of different periods involving a person’s physical, emotional, and intellectual activity, as used to explain or predict behavior.