Although there is considerable interindividual and intercountry variability, levels of physical activity and physical fitness tend to be higher in the summer than in the winter months. The specific element of activity that varies the most seems to be the duration of an individual exercise session, but it is unclear whether this is caused by responses to the longer daylight hours or higher environmental temperatures that occur in the summer. There may be chronic and acute effects of this seasonality on health. It is difficult to identify seasonal variations in maximal oxygen consumption and other laboratory tests of exercise performance with well-trained athletes, despite obvious changes in their competitive performances throughout the year. This obstacle might be the result of the relative lack of measurement sensitivity in laboratory indices of performance, and the fact that athletes remain relatively active, even outside of their competitive seasons. In the absence of longitudinal studies (ie, over 2 to 3 years) on humans who are completely isolated from seasonal changes in the environment, it is difficult to make firm conclusions regarding the relative endogeneity of seasonal rhythms. Most seasonal changes that have been found in human physiology can be explained just as logically by exogenous factors (eg, seasonal responses to environment and activity) as by any seasonal biological clock, although there is a definite suggestion that a given level of exercise in the winter might lead to more favorable changes in body composition. In athletes, seasonal physiological changes generally reflect the amount of training that is performed, although individual athletes may have their training cycles disrupted by the influence of environmental seasonal fluctuations in temperature, humidity, or injury and illness.