Physical fitness, exercise, age and human sleep

A number of studies have reported that physically fit individuals have more slow wave sleep (SWS) than unfit individuals on nights following non-exercise days. However these studies have failed to control for residual exercise effects from previous days. The present study was designed to replicate the original finding and to determine if the reported difference was due to a chronic fitness effect or to a residual exercise effect. In addition the fitness effect was tested in an older population than had previously been tested. The design consisted of two levels of fitness, two age groups (22-0 and 31-8 years of age) and 3 measurements occasions, the first following afternoon exercise and the second and third following 2 and 4 days of exercise deprivation for fit subjects and 3 non-exercise days for unfit subjects. The conditions were arranged in a 223 design. The results indicated a significant effect of physical fitness on SWS in the younger age group but not the older. As there was no immediate effect of exercise, the elevation of SWS in the younger fit group appears to represent a chronic difference between young fit and unfit populations. The results have implications for the nature of restorative processes occurring during sleep.