Physiological significance of bright vs. dim light intensities during the daytime for thermoregulatory responses, digestive functions and evening dressing behavior in the cold

Abstract

The present paper consists of 6 sections; (1) temporal variability of preferred illuminance self-selected by women whilst awake; (2) core body temperature under the influence of bright/dim light during the daytime; (3) the administration of exogenous melatonin during the daytime lowers the thermoregulatory set-point in humans; (4) the effects of exposure to bright or dim light during the daytime on digestive activity in humans; (5) the influence of different light intensities during the daytime on evening dressing behavior in the cold; and (6) concluding remarks. These sections emphasize how deeply human thermoregulatory responses, including behavioral, cold/warm sensory and autonomic responses, are under the control of light intensities during the daytime. The physiological basis for this is that the thermoregulatory set-point is variable under the influence of different light intensities during the daytime, due to probable involvement of the hormone melatonin. The results strongly suggest that we should take the strict control of surrounding illuminance into account when we study the effects of ambient temperature on human thermoregulatory responses. For example, the sensation of cold by a subject may vary even when subjected to identical thermal (cold) stimuli, depending on previous experience of different light intensities. This is because the thermoregulatory set-point may be influenced differently by the surrounding illuminance.