Biological rhythms are molecular, physiological, or behavioral events that recur and are a ubiquitous property of all living organisms. Rhythmicity coordinates biological systems and synchronizes them with the external environment. Ultradian rhythms are of a short duration (seconds, minutes, or hours), while circadian rhythms have a period of about 1 day. Two well-studied examples are the 40-min ultradian rhythm of cellular respiration in yeast and the mammalian master circadian clock in the brain. Disruptions of circadian rhythms are of importance in pathology, including cancer, sleep disorders, and mental health. Ultradian and circadian clocks may share common elements at the molecular level.