Investigation of circadian rhythms in metabolic responses to exercise

This study attempted to determine circadian rhythms in metabolic responses to submaximal and maximal workloads by concentrating on repeated observations of a male subject over 16 weeks. Altogether 36 experimental tests were undertaken on a cycle ergometer, six at each of the times, 0200, 0600, 1000, 1400, 1800 and 2200 hours. The work test comprised two consecutive 5-min periods at 82 and 147 W, followed immediately by an incremental workload to exhaustion. Significant circadian rhythms were found at rest pre-exercise for pulse rate (fh), VO2, VE and rectal temperature (Tr) (p<0·001), with fH leading the others in phase and Tt lagging significantly behind VE (p<0·05). The circadian cycle in VE persisted at both submaximal steady rates, with the VO2 significant at the higher rate only when uncorrected for body weight. There was no evidence of a circadian variation in ‘muscular efficiency’. No significant rhythms were found in metabolic measures at maximal exercise or during recovery, indicating no time of day effect in the physiological capacity for exercise. The circadian variation in Tr persisted in phase and amplitude under maximal conditions and during recovery, indicating that the thermal load induced by a set exercise regime is independent of time of day.