Rodahl, A., O'Brien, M., & Firth, P. G. (1976). Diurnal variation in performance of competitive swimmers. Journal of Sports Medicine and Physical Fitness, 16, 72-76.

Swimmers performed significantly faster in the late afternoon than early in the morning. Evening practice swims should be expected to be faster than those at a morning practice. [Baxter, C., & Reilly, T. (1983). Influence of time of day on all-out swimming. British Journal of Sports Medicine, 17, 122-127.]

When performances were measured at five different times of the day between 6:00 and 22:00 hrs, a steady improvement was demonstrated. [Sinnerton, S., & Reilly, T. (1991). Effects of sleep loss and time of day in swimming. In D. Maclaren, T. Reilly, A. Lees, & M. Hughes (Eds.), Biomechanics and medicine in swimming VI. London: E. and F. N. Spon.]

Swimmers performed better in the evening at 17:30 hrs than in the morning at 6:30 hrs. Front crawl improved by 3.6% over 400 m and 1.9% over 100 m. [Reilly, T., & Marshall, S. (1991). Circadian rhythms in power output on a swim bench. Journal of Swimming Research, 7, 11-13.

Circadian variation in power output (as performed on a biokinetic swim bench) and its relation to circadian rhythms in body temperature and subjective alertness were measured in 14 competent swimmers at six equidistant times (starting at 2:00 hr) of the day. Peak and mean power on a 30 s test were noted. Subjective scale responses for alertness and pre-exercise rectal temperatures were recorded.

Distinctive circadian rhythms for pulse rate, rectal temperature, alertness, and both power measures were exhibited. The general time for the peak values was after 16:00 hr. The difference between the highest and lowest values in the rhythm was 14% for mean power and 11% for peak power. It was suggested that the amplitude of circadian rhythms increases with the complexity of motor tasks because of the size of the values obtained. Complex activities are affected to a greater degree by circadian rhythms that are simple activities.

The circadian rhythm in power output on a swim bench was closely related to the rhythm for body temperature and alertness. The existence of these rhythms should be taken into account when planning strength and power training stimuli.

Another implication of this finding is that comparative performances and test results must be gathered at the same time of day. Otherwise, test differences could solely be the result of circadian predispositions.

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