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This study examined the existence of circadian rhythms in rectal temperature (Tr) and selected skin temperatures at rest and during light (82 W) and medium (147 W) exercise on a cycle ergometer. Each intensity was sustained for 5 min and then followed by a graded exercise test to exhaustion. The Tr and skin temperatures were also measured on cessation of exercise and 4 min post-exercise. Fifteen males participated, measurements covering six interspersed times of day. The mean exercise time to exhaustion of 19.83 min was independent of the time of day and caused a constant rise of 0.7 degree C in Tr. Significant rhythms were observed in Tr, Tc (chest), and Ta (arm) under all conditions: findings for Tl (leg) were nonsignificant at maximal exercise and post-exercise (P greater than 0.05). The crest time for Tl had a significant lead over that of Tr and the other skin sites which were in phase concordance, mean difference in crest times being 4.73 h at rest and 4.42 h during submaximal exercise. The results demonstrated constancy with time of day in the thermal load imposed by fixed exercise. This was reinforced when mean skin and mean body temperatures were computed. The general pattern did not apply to the exercised limb, the pre-exercise rhythm disappearing after 10 min exercise. Differential heat conductances according to skin sites are implicated in the phase differences between the leg and the other temperature sites.
Reilly et al. (Mon,) studied this question.