Circadian biology and exercise: the time to move.

Exercise performance in endurance- and power-based events is time-of-day dependent in both humans and rodents. Accordingly, there has been growing interest in determining whether there is an optimal time of day for physical activity that can amplify the well-known benefits of exercise on metabolic health in humans. Here, we discuss critical features of circadian biology that underpin many of the physiological responses to the timing of exercise. Recent studies indicate that the circadian clock regulates exercise capacity through the coordination of tissue-specific physiological responses, including fuel metabolism and mitochondrial biogenesis. Synchronized actions between circadian clocks and clock-output pathways residing in the skeletal muscle and other tissues are likely to explain how external time-of-day cues influence exercise performance and physiological responses to exercise. Understanding the circadian biology of exercise will provide the foundation on which future individualized exercise protocols are prescribed to improve metabolic health outcomes at both individual and population levels.