Many biological functions change cyclically over a 24-h period, such cycles being referred to as circadian rhythms. The major rhythms of relevance to examine performance are those of body temperature and the sleep-wake cycle. Many components of exercise performance are closely related to the body temperature curve which peaks in the early evening. Exercise with predominantly neuromotor and cognitive components depend also on the underlying sleep-wake cycle. Some performance measures are subject to ultradian cycles and show a transient decline in the early afternoon. Optimal time of day for exercise is determined not just by endogenous rhythms but also by the nature and intensity of exercise, the population concerned, environmental conditions, and individual phase types. Environmental factors impinging on circadian rhythms include light, heat, air ionization, activity and eating patterns, and social activities. Endogenous rhythms are desynchronized when perturbed by nocturnal shift work or time-zone transitions. Coping with desynchronosis involves behavioral, dietary, or pharmacological treatments. Sleep loss interacts with circadian rhythmicity but affects cognitive function more so than gross motor actions. The existence of self-sustaining rhythms should be recognized by athletic practitioners, sports scientists concerned with experimental work and fitness testing, sports injury specialists, and sports organizers concerned with the travel plans of athletes.