Day-night difference in the preferred ambient temperature of human subjects.

To investigate the existence of a day-night difference in thermal preference, 32 college students (16 male, 16 female) were asked to indicate a threshold of discomfort in a climate-controlled environment. The experiment was performed at two distinct times of day that corresponded to the peaks and troughs of the subject's circadian rhythm of body temperature. Males, but not females, felt comfortable at a higher ambient temperature during the trough than during the peak of the body temperature rhythm. These data support the hypothesis (derived from animal studies) that behavioral thermoregulation opposes the circadian rhythm of body temperature.