Effect of a 15 minute light pulse on nocturnal serum melatonin levels in human volunteers.

Monitoring the daily melatonin rhythm during the course of phototherapy for affective disorders may be beneficial in assessing the efficacy of such treatments. It is therefore of interest to study the effects of the timing, duration, and intensity of bright light pulses on melatonin levels in normal subjects. To examine the effects of a single exposure to a brief burst of bright light on serum melatonin, groups of healthy human volunteers of both sexes were treated with a 15 minute pulse of bright light (350 cd/m2) early in the evening during the winter months. Serial blood samples were collected from each person and the effect of the light pulse on serum melatonin and cortisol levels determined. Melatonin levels were significantly but only transiently suppressed by the light pulse, while cortisol levels were not affected. These results demonstrate that short duration bright light treatments can influence the melatonin rhythm generating system in humans.