ONE-HOUR EXPOSURE TO MODERATE ILLUMINANCE (500-LUX) SHIFTS THE HUMAN MELATONIN RHYTHM

Salivary melatonin levels were measured in 12 healthy volunteers in or der to determine whether a moderate light intensity, which suppresses the nocturnal rise of melatonin, was able to shift the melatonin rhyth m. The samples were collected at 1-hr intervals under lighting of < 10 0 lux (experiment 1) or < 10 lux (experiment 2). The control melatonin profiles were determined during the first night. In the second night the subjects were exposed to light of 500 lux for 60 min during the ri sing phase of melatonin synthesis. The third series of samples was col lected during the third night. The mean decrease of melatonin levels b y the exposure to light was 56% of the prelight concentrations. The me latonin onset times were delayed significantly (about 30 min) the nigh t after the exposure to light. The melatonin offset times tended to be delayed in experiment 2. The shifts of the melatonin offset correlate d positively with the amount of the melatonin suppression. The results suggest that a relatively small and short lasting light-induced inter ruption of melatonin synthesis may affect the melatonin rhythm in huma ns.