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 w ere exposed to an ambient temperature (T-a) of 42 degreesC and relative hum idity (RH) of 40% for 4 h (14:00- 18:00 h)/day for 9-10 consecutive days. R ectal temperature (T-re) was measured, and T-pref was determined at two dis tinct 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-c only in the PM test. There was no difference i n the T-pref between the two tests in the control condition. However, T-pre f in the PM test was significantly lower than that of the AM test in the he at-acclimated condition. The findings suggest that repeated heat exposure i n humans for 4 h at a fixed time daily alters the core temperature level an d behavioral thermoregulatory function, particularly during the period when the subjects had previously been exposed to heat. (C) 2001 Elsevier Scienc e Inc. All rights reserved.