Circadian rhythms are endogenously generated rhythms with a period length of about 24 hours. A biologic clock in the hypothalamic suprachiasmatic nuclei is responsible for the generation of circadian rhythms. Notable examples of the circadian rhythms include the sleep-wake cycle and rhythms in hormone production. Abnormalities of the circadian system include biologic clock lesions that result in arrhythmic behavior and irregular sleep patterns. Abnormalities of the circadian system also occur when there is desynchronization of clock phase with that of the outside world, resulting in conditions such as “jet-lag.” Numerous aspects of human physiology are greatly influenced by the time of day, as is the pathogenesis of illness. During development, the circadian system becomes functional at early stages and is regulated by photic information. With the continued elucidation of circadian system influences on human physiology and illness, it is anticipated that circadian biology will have an increasingly important impact on the clinical care of children.