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An investigation along the circadian 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 5-min loads at 82 and 147 W, then a graded increase of work-rate every 2 min to exhaustion. Significant 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 less than or equal to 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.
Reilly et al. (Mon,) studied this question.