Eight physically trained subjects underwent three experimental conditions on separate occasions between 1400 and 1800 h. Two conditions acted as controls for a high-intensity exercise (HI) condition of treadmill running at 80% VO2 max for a total of 80 min. The rate of body heating was modelled in a no-exercise passive heating condition (PH), and the total exercise load was replicated in a low-intensity condition (LI) at 40% VO2 max for 160 min. LI produced no slow-wave sleep (SWS--stages 3 + 4 sleep) changes, but was the only condition to produce significant increases in sleep length and in non-rapid eye movement (REM) sleep (stages 1 + 2 + 3 + 4), and a significant decrease in sleep onset time. Although HI and PH produced similar SWS increases, these consisted of significant increases in stage 3 sleep for HI and in stage 4 sleep for PH. No REM sleep parameter was affected under any condition. Self-estimates of presleep tiredness produced no significant findings. It was concluded that a high and sustained rate of body heating for 1-2 h, particularly the inherent rapid rates of core temperature increase and of body dehydration, may trigger a SWS response, and that exercise may simply be a vehicle for these effects.
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