Why does circadian rhythm affect performance?
There are a number of possible explanations as to why performance may be enhanced during the hours around the peak of the circadian rhythm, but increased core temperature almost certainly plays a major role. Higher body temperatures result in less viscous blood flow and muscles that are more supple, with less energy loss from internal friction. However, there is evidence that increased core temperatures in the afternoon/evening as a result of the circadian rhythm may also help because the body is in more of a ‘heat loss’ mode than compared with early morning ‘heat gain’ mode when core temperatures are low. British researchers looked at heart rate, core temperature, sternum skin temperature and forearm skin blood flow during exercise, and throughout a subsequent 30-minute recovery period in 12 males exercising at 70% VO2max at both 08.00h and 18.00h(13). Comparisons were made of the changes of heart rate, temperature, and skin blood flow produced by the exercise at the two times of day.
