Preferred illuminance self-selected by women under the influence of face cooling

The aim of the study was to investigate the illuminance selected in the morning by subjects whose face was cooled and whose tympanic temperature has been lowered by 0.15-0.25 degrees C. The hypothesis tested was that a difference between the actual core temperature and its set point (load error) will influence the preferred illuminance. Specifically, it was hypothesised that a greater load error would lead to the choice of a higher light intensity because this would suppress the melatonin secretion more effectively and thus accelerate the morning rise of core temperature. Seven females served as participants. They were required to rise in the morning 3 h after their minimum rectal temperature, as judged from measurements of rectal temperature made during sleep. The subjects sat 1.5 m away from a lighting box and every 15 min were instructed to choose the level of illuminance (light intensity) they preferred by manipulating a lever in the control box. This procedure was performed twice, in the face-cooling and no-face-cooling condition. The major finding was that a higher light intensity was preferred when the face was cooled down by 0.15-0.25 degrees C, suggesting that an increased load error between actual core temperature and its set point may play a behavioural role in the selection of preferred illuminance.