Kline, C. E., Youngstedt, S. D., Devlin, T. M., Lee, A. Y., Zielinski, M. R., Moore, T. A., Davis, M. J., & Durstine, J. L. (2006). Circadian variation in swim performance. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1543.

This investigation tested whether there is an internally-generated circadian rhythm in swimming performance that is independent of environmental and behavioral factors. Experienced swimmers (N = 25) were assessed for 50 consecutive hours in a laboratory setting. Ss adhered to a three-hour ultra-short sleep/wake cycle, involving two hours of wakefulness in dim light followed by one hour of sleep in darkness, repeated throughout the 50-hour period. The protocol distributed sleep/activity, posture, caloric intake, etc., equally across the 24-hourr period. Ss performed six maximal-effort 200-m swimming trials. The trials began at one of eight randomly assigned times of day, and were each separated by nine hours. There were 147 swim trials distributed around the clock. Tympanic membrane temperature was recorded three times during each awake-period. A cosine fit was used to establish the minimum body temperature. Swimming performances were z-transformed and expressed relative to minimum body temperature. Performances were then placed into two-hour bins equally spaced relative to minimum body temperature.

There was a significant circadian pattern in swimming performance. Performance peaked between five and seven hours before the minimum body temperature (corresponding to the time of ~2300 hr) and was worst between one hour before and one hour after the minimum body temperature (corresponding to ~0500 hr). The variation from peak to worst performance was approximately 4 seconds.

Implication. There is a circadian rhythm in athletic performance, independent of environmental and behavioral masking effects. This phenomenon could influence performances at different times of day, and would be particularly disruptive following substantial travel that resulted in time shifts.

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