An Introduction to Circadian Endocrine Physiology: Implications for Exercise and Sports Performance

Abstract

Biological rhythms that recur approximately every 24 h (circadian) are driven by the internal biological clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. These circadian rhythms are intrinsically generated but also synchronized by external time cues—the daily light/dark cycle being the most important. Preferably under controlled conditions (constant routine or forced desynchrony), appropriately timed measurement of the 24-h variation in certain hormones (e.g., melatonin) or other relevant physiological variable (e.g., core body temperature) can be used to evaluate circadian phase. Interactions between circadian rhythms and sleep have a significant effect on multiple physiological processes. Human performance (cognitive and athletic) exhibits a circadian variation with minimal levels around time points close to the core body temperature minimum (shortly after the maximum melatonin secretion) while peak performance is observed around time points close to the core body temperature maximum (shortly before melatonin onset). Furthermore, misalignment of the internal clock with the external environment as seen in jet lag and shift work can negatively affect athletic performance and appropriately timed exposure to and avoidance of bright light can be used to mitigate the effects of jet lag. The circadian variation in performance can affect outcome of competitions and could be also important for choosing the optimal time to exercise.