The energy conservation model proposes that the main function of sleep is to lower metabolic requirements periodically and thus to conserve energy. However, certain variations in energy expenditure, such as that produced by physical exercise, have not been found to be consistently related to sleep length. We hypothesized that, because sleep variables may adapt relatively slowly to metabolic changes, the effect of exercise on sleep time would be observed as a function of habitual exercise patterns, not of daily variations. The study consisted of a retrospective analysis of five experiments. Although the design of each experiment was idiosyncratic, all involved physically fit and/or unfit subjects whose sleep was assessed following daytime exercise and/or no exercise conditions. As predicted, fit subjects slept significantly longer than unfit subjects, and daytime exercise had no consistent effect on sleep duration. However, for several reasons, the relevance of the data to the energy conservation model is uncertain.