Bright light exposure of a large skin area does not affect melatonin or bilirubin levels in humans

Background: Light treatment through the eyes is effective in alleviating the symptoms of some psychiatric disorders. A recent report suggested that skin light exposure can affect human circadian rhythms. Bilirubin can serve as a hypothetical blood-borne mediator of skin illumination into the brain. We studied whether bright light directed to a large body area could suppress the pineal melatonin secretion or decrease serum total bilirubin in conditions that could be used for therapeutic purposes. Methods: Seven healthy volunteers participated in two consecutive overnight sessions that were identical except for a light exposure on the chest and abdomen in the second night from 12:00 to 6:00 (10,000-lux, 32 W/m2 cool white for six subjects and 3000-lux, 15 W/m2 blue light for one subject). Hourly blood samples were collected from 7:00 to 7:00 for melatonin radioimmunoassays. Bilirubin was measured by a modified diazo method in blood samples taken at 12:00 and 6:00 and in urine samples collected from 7:00 to 11:00 and from 11:00 to 7:00 . Results: The skin light exposure did not cause any significant changes in serum melatonin or bilirubin levels. The excretion of bilirubin in urine was also the same in both sessions. Conclusions: Significant melatonin suppression by extraocular light does not occur in humans. Robust concentration changes of serum total bilirubin do not have a role in mediating light information from the skin to the central nervous system.