Circadian rhythm of thermoregulating responses in man exposed to thermal stimuli

Circadian changes in man's thermoregulating responses were studied in two experimental runs during which 7 volunteer subjects were exposed at 8 equally spaced time points of the 24-h span to a quick rise in temperature (To = 40 degrees C, v = 1 m/sec, P H2O less than 1 kPa) or to progressive cooling, the final temperature being reached after 45 min (To = 20 degrees C, v = 0.2 m/sec, P H2O less than 1 kPa). Under heat exposure, a circadian rhythm was observed in the period preceding the onset of sweating, and also in rectal temperature and mean body temperature, both at the onset of sweating and at its steady state. The time constant of the sweating rate and the sweating rate at the steady state show no apparent changes. Under cold exposure there is no detectable circadian rhythm in the mean body temperature at the onset of shivering, however, a circadian evolution appears in rectal temperature at the onset of shivering and at the end of the experiment. A circadian rhythm also appears for the metabolic response at the end of the experiment. These results suggest the presence of a circadian rhythm of the set point with no change in the gain of sweat responses. They also imply that thermoregulating responses to cold follow circadian changes.