Any periodic, more-or-less regular fluctuation or cycle in a biological system or process that is not wholly under the control of environmental cues but is controlled centrally by a biological clock. It may be an ultradian rhythm such as a circannual rhythm or a menstrual cycle; a circadian rhythm such as the sleep-wake cycle; or an infradian rhythm such as an alpha wave, basic rest-activity cycle, beta wave, delta wave, gamma wave, sensorimotor rhythm, or theta wave. See also chronobiology, pacemaker neuron, suprachiasmatic nucleus, Zeitgeber. Compare biorhythm.
Subjects: Medicine and Health.