Ambient temperatures preferred by humans acclimated to heat given at a fixed daily time

We investigated preferred ambient temperatures ( T pref) of heat-acclimated humans to assess their behavioral thermoregulation. Seven male volunteers were exposed to an ambient temperature ( T a) of 42°C and relative humidity (RH) of 40% for 4 h (14:00–18:00 h)/day for 9–10 consecutive days. Rectal temperature ( T re) was measured, and T pref was determined at two distinct times of day, 09:00–11:00 h (AM test) and 14:00–16:00 h (PM test), in both heat- and nonheat-acclimated (control) conditions. Heat acclimation significantly decreased T re only in the PM test. There was no difference in the T pref between the two tests in the control condition. However, T pref in the PM test was significantly lower than that of the AM test in the heat-acclimated condition. The findings suggest that repeated heat exposure in humans for 4 h at a fixed time daily alters the core temperature level and behavioral thermoregulatory function, particularly during the period when the subjects had previously been exposed to heat.