Physical Fitness, Exercise, and REM Sleep Cycle Length

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Abstract
It has been suggested, on the basis of phylogenetic and ontogenetic data, that the length of the REM sleep cycle is negatively related to metabolic rate. However, experimental studies have not consistently found this relationship. We report the effects of two variables, the level of physical fitness and the amount of daytime exercise, on sleep cycle length. Since both physical fitness and exercise involve increased levels of energy expenditure, and thus increased metabolic rate, it was predicted that they would he negatively related to cycle length. The sleep of 46 subjects (20 fit and 26 unfit) was assessed on nights following both exercise and non‐exercise conditions. The cycle length of fit subjects was found to be shorter than that of unfit subjects. However cycle length on a night following daytime exercise did not differ from that on a night following a no‐exercise day. We conclude that sleep cycle length is responsive to relatively long term variations in energy expenditure, but not day to day changes.

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