EXOGENOUS MELATONIN FAILS TO COUNTERACT THE LIGHT-INDUCED PHASE DELAYOF HUMAN MELATONIN RHYTHM

Salivary melatonin levels were measured in 6 healthy volunteers in ord er to determine whether the phase shift caused by a single 60-min ligh t pulse of 2000 lux might be inhibited by maintaining high melatonin c oncentration. In the control sessions, the samples were collected at 6 0-min intervals under lighting of < 10 lux from 18.00 to 11.00 h. In t he light-exposure sessions, placebo or 0.5 mg melatonin was administer ed orally 60 min prior to the light pulse, timed at the rising phase o f 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 maintenan ce of high melatonin levels during the light exposure did not countera ct the influence of bright light on the melatonin rhythm. Thus, in the adjustment of the melatonin rhythm, light is a stronger regulator tha n melatonin itself.