Kline, C. E., Durstine, J. L., Davis, J. M., Moore, T. A., Devlin, T. M., Zielinski, M. R., & Youngstedt, S. D. (2007). Circadian variation in swim performance. Journal of Applied Physiology, 102, 641-649.

The purpose of this study was to examine whether there is a circadian rhythm in swim performance that is independent of masking factors (e.g., sleep, ambient temperature, and energy intake). Experienced swimmers (N = 25) were assessed for 50–55 consecutive hours in the laboratory. Ss followed a 3-hour "ultra-short" sleep-wake cycle, involving one hour of sleep in darkness and two hours of wakefulness in dim light throughout the observation. The protocol distributed behavioral and environmental masking factors equally across the 24-hour period. Each S performed six maximal-effort 200-m swim trials that were distributed equally across eight times of day (147 trials). Each trial was separated by 9 hours.

A significant pattern in swim performance relative to environmental and circadian times of day was revealed. Performance peaked 5–7 hours before 11 PM and was worst from one hour before to one hour after 5 AM. Mean swim performance was 169.5 seconds; circadian variation from peak to worst performance was 5.8 seconds. These data suggest a circadian rhythm in athletic performance independent of environmental and behavioral masking effects.

Implication. Swimmers are likely to perform better at evening practices and competitions when compared to what is possible in early morning swimming sessions.

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