Exogenous melatonin fails to counteract the light-induced phase delay of human melatonin rhythm

Salivary melatonin levels were measured in 6 healthy volunteers in order to determine whether the phase shift caused by a single 60-min light pulse of 2000 lux might be inhibited by maintaining high melatonin concentration. In the control sessions, the samples were collected at 60-min intervals under lighting of < 10 lux from 18.00 to 11.00 h. In the light-exposure sessions, placebo or 0.5 mg melatonin was administered orally 60 min prior to the light pulse, timed at the rising phase of the melatonin synthesis. The after-light sessions, one day after the light exposure, were like the control sessions. The average delays of the melatonin half-rise and half-decline times were equal (about 0.7 h) in the placebo and melatonin replacement experiments. The maintenance of high melatonin levels during the light exposure did not counteract the influence of bright light on the melatonin rhythm. Thus, in the adjustment of the melatonin rhythm, light is a stronger regulator than melatonin itself.