Temporal variability of preferred lighting conditions self-selected by women

The purpose of this study was to know what quantity and quality of illuminance during the waking period is selected by subjects and to discuss these in terms of circadian rhythm of core temperature. Five females served as subjects. They were instructed to choose the intensity and color temperature of the illumination they preferred every 15 min during wakefulness by adjusting a light box. The illuminance selected rose during the course of the first 5 h and the color temperature rose over the first 2 h that they waked. Both parameters were decreased steadily over the 5 h before retiring. When the chosen illuminances and color temperatures were plotted against core temperatures during these two periods, it was found that both of these parameters were higher for identical core temperature in the morning after rising than in the evening before retiring. This behavior could accelerate the morning rise of core temperature (at the time when the set point is rising) and the fall of core temperature in the evening (at the time when the set point is falling).