Exercise has generally been held to result in an increase in slow wave sleep (SWS) and sleep duration. We report the results of three experiments which assessed the effects of exercise on the sleep of young fit women as compared to young fit men; fit older men (aged 41 years) as compared to fit younger men (aged 22 years); and the effects of power exercise on the sleep of a group of trained power lifters. In each experiment physically fit subjects had their sleep assessed following both late afternoon exercise and no exercise conditions. In none of the experiments did exercise result in an increase in SWS or sleep duration. Instead the tendency was for exercise to affect sleep adversely.