The primary purpose of this study was to determine the effect of bright light exposure during the daytime on dressing behavior in the cold. Seven female volunteers were exposed to bright light of 4,000 lx ("Bright") or dim light of 10 lx ("Dim") from 10 h to 18 h, complete darkness during the sleep period (22:30 h-06:00 h), and 10 lx for the rest of the time. The subjects were instructed to dress to remain comfortable when the ambient temperature was decreased from 30 degrees to 15 degrees C (20:30 h-22:30 h). Most subjects dressed more quickly and with thicker clothing in the Dim condition and felt cooler during the last 30 min of the temperature fall. The rectal temperature showed clear circadian rhythm both under Bright and Dim conditions, but it was significantly lower during sleep in the Bright condition. It is suggested that the set-point of core temperature is reduced at night during the Bright condition.