The clockwork of champions: Influence of circadian biology on exercise performance

Exercise physiology and circadian biology are distinct and long-standing fields. Recently they have seen increased integration, largely due to the discovery of the molecular components of the circadian clock and recognition of human exercise performance differences over time-of-day. Circadian clocks, ubiquitous in cells, regulate a daily tissue specific program of gene expression that contribute to temporal patterns of physiological functions over a 24-h cycle. Understanding how circadian clock function in skeletal muscle, as well as other tissues contribute to exercise performance is still in the very early stages. This review provides background on this emerging field with a review of early exercise and time-of-day studies in both human and animals. We then move into the role of the circadian clock and its daily program of gene expression in skeletal muscle with a focus on specific metabolic and physiological outputs that vary over time-of-day. Lastly, we discuss the recognition that the timing of exercise communicates with the skeletal muscle circadian clock to adjust its phase settings and why this maybe important for performance and health. • The Circadian clock is a molecular mechanism temporally orchestrating mammalian physiological functions on a 24-h cycle. • Exercise performance exhibits diurnal patterns, which peaks in the late afternoon/evening (16:00–19:00 h). • The timing of exercise serves as a zeitgeber, shifting clock phase to promote alterations in metabolism and clock output. • Exercise-induced phase shifts in the muscle clock occur independently and do not affect SCN circadian rhythms.