Thermoregulation During Mild Exercise at Different Circadian Times

Eight healthy subjects exercised at 90watts on a cycle ergometer on four occasions, at times close to the minimum, maximum rate of rise, maximum, and maximum rate of fall of their resting core temperature. The duration of exercise was determined by the time taken for the core (rectal) temperature to reach an equilibrium value. Forearm skin blood flow and temperature were measured regularly during the exercise, as were heart rate and ratings of perceived exertion. Sweat loss was calculated by weighing the subjects nude before and after the exercise. The rise of heart rate was not significantly different at the four times of exercise, though the rating of perceived exertion was greatest at 05:00 h. Resting core temperatures showed a significant circadian rhythm at rest (the timing of which confirmed that exercise was being performed at the required times), but the amplitude of this rhythm was decreased significantly by the exercise. The initial rate of rise of core temperature, and the total rise from the resting to the equilibrium value, were both inversely proportional to resting temperature. The time-course of the rise was accurately described by a negative-exponential model, but this model gave no evidence that the kinetics of the equilibration process depended upon the time of day. The thermoregulatory responses to the rise in core temperature--the amount of total sweat loss and rises in forearm skin blood flow and temperature--differed according to the time of exercise. In general, the responses were significantly greater at 17:00h compared with 05:00h, and at 23:00 h compared with 11:00 h. The results accord with predictions made on the basis of previous work by us in which core temperature rhythms have been separated into components due to the endogenous body clock and due to the direct effects of spontaneous activity. The results are discussed in terms of the ecological implications of the differing capabilities of humans to deal with heat loads produced by spontaneous activity or mild exercise at different phases of the circadian rhythm of resting core temperature.