n. a normal biological fluctuation which occurs in an organism in response to periodic, cyclic changes in its internal environment. These fluctuations regularly occur on the physiological and psychological level, and they involve a change in energy level, hormonal level, and sexual desire. Also called biorhythm- internal rhythm. See endogenous rhythm- also circadian rhythm.
BIOLOGICAL RHYTHM: "There are periodic changes which affect a being's physiological and psychological functions, and these variations occur in a cycle known as the biological rhythm."