Circadian Rhythms and Exercise

Circadian rhythms are self-sustaining phenomena with a periodicity of approximately 24 hours (Latin: circa = about, diem = 1 day). For example, the human circadian rhythm of body temperature shows a cosine wave pattern, typically peaking in the early evening (57 p.m.) and reaching a nadir in the early morning (46 a.m.). This entry will review evidence that exercise can influence the circadian system and discuss implications for health and performance.

Every living organism displays circadian rhythms, which probably evolved to allow adaptation to the 24-hour rotation of the earth. The bilaterally coupled suprachiasmatic nuclei (SCN) of the hypothalamus are the predominant anatomic locus of the mammalian circadian pacemaker. Neural and humoral outputs from the SCN to other centers in the hypothalamus and endocrine system drive multiple behavioral and physiological rhythms.

In humans, direct measurement of SCN activity is essentially impossible, so measurement of the human circadian system involves assessment of the output markers, such as body temperature and melatonin excretion, which can be considered the “hands of the clock.” The measurement of these rhythms can be partly confounded or “masked” by environmental and behavioral factors, such as energy intake, activity, temperature, and light. For example, the diurnal rhythm of body temperature observed under normal conditions is influenced partly by day-night fluctuations in physical activity and food intake.

However, by definition, these rhythms are endogenously produced, though their measurement is influenced by these masking factors. That is, the rhythms persist even when an organism is separated from time-of-day cues or from diurnal fluctuations in these masking factors. The earliest human studies demonstrated this point in cave experiments, where there was no fluctuation in temperature and light and participants were isolated from the environment. Modern laboratory techniques have been developed to “unmask” circadian rhythms from masking factors. These include the constant routine, forced desynchrony, and ultrashort sleep-wake cycle. In the 180-minute ultrashort sleep-wake schedule, participants are allowed 60 minutes for sleep in darkness (<1 lux), followed by 120 minutes of out-of-bed wakefulness in dim light (30 lux). This schedule is repeated round the clock for up to 10 days and distributes masking effects equally around the 24-hour day.