Selective Persistence of Circadian Rhythms in Physiological Responses to Exercise

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Abstract
An investigation along the cicadian time scale in heart rate (HR), rectal temperature (Tr)and metabolic functions at rest, during submaximal and maximal exercise, and post-exercise was conducted using 15 male subjects. Each individual performed a cycle ergometer test on six separate days, temporally placed over the solar day to incorporate equidistant observation points. The exercise tests involved two consecutive S-min loads at 82 and 147 W, then a graded increase of work-rate every 2 m in to exhaustion. Significa nt circadian rhythms were evident at rest for HR, Tr, vO2 and vE, the HR acrophase at 13:50 hr leading that of the Tr and metabolic variables significantly (P≥0.05). The circadian rhythms in metabolic measures were not evident at any exercise level or post-exercise and muscular efficiency was found to be independent of time of day. The exercise tolerance time and blood lactate post-exercise values failed to show a significant rhythm. In contrast the rhythms in HR and Tr were retained with similar phase and amplitude to those at rest during all work rates and post-exercise, though the amplitude of the HR cycle was attenuated at maximum.