A comparison of the immediate effects of moderate exercise in the early morning and late afternoon on core temperature and cutaneous thermoregulatorymechanisms

Twelve healthy male subjects each undertook two bouts of moderate exercise (70% VO(2)max for 30 minutes) in the morning (08:00) and late afternoon (18 :00) at least 4 days apart. Measurements were made of heart rate, core (rec tal) temperature, sternum skin temperature, and forearm skin blood flow dur ing baseline conditions, during the bout of exercise, and throughout a 30-m inute recovery period. Comparisons were made of the changes of heart rate, temperature, and skin blood flow produced by the exercise at the two times of day. Student t tests indicated that baseline values for core temperature (37.15 degrees C +/- 0.06 degrees C vs. 36.77 degrees C +/- 0.06 degrees C ) and sternum temperature (33.60 degrees C +/- 0.29 degrees C vs. 32.70 deg rees C +/- 0.38 degrees C) were significantly (p < .05) higher in the late afternoon than the early morning. Two-way analysis of variance (ANOVA) indi cated that the increases in core and sternum temperatures during exercise w ere significantly less (p = .0039 and .0421, respectively) during the after noon bout of exercise compared with the morning, even though the work loads , as determined by changes in heart rate, were not significantly different (p = .798) at the two times of testing. There were also tendencies for rest ing forearm skin blood flow to be higher in the afternoon than in the morni ng and for exercise to produce a more rapid rise in this variable in the af ternoon. The possible mechanisms producing these responses to exercise are discussed in terms of those that are responsible for the normal circadian r hythm of core temperature. It is concluded that the body's ability to remov e a heat load is less in the early morning, when the circadian system is in a "heat gain" mode, than in the late afternoon, when heat gain and "heat l oss" modes are balanced more evenly.