Midday exposure to bright light changes the circadian organization of plasma melatonin rhythm in humans.

Effects of bright light exposure at midday were examined on plasma melatonin rhythm in humans under controlled living conditions. Bright light of 5000 1x was provided from the ceiling at midday (1100-1700 h) for 3 consecutive days and the circadian rhythm in plasma melatonin was determined from the fourth to fifth day. The control study was performed in the same subjects who spend four days under dim light conditions (less than 200 1x). The subjects were allowed to sleep from 2400 to 0800 h. The onset phase, but not the end phase, of plasma melatonin rhythm was significantly phase-advanced by bright light exposure. Furthermore, the area under the curve of nocturnal melatonin rise was significantly larger under bright light exposure than under dim light. These findings indicate that midday exposure to bright light for 3 consecutive days changes the circadian organization of plasma melatonin rhythm in humans.