RAPID PHASE ADVANCE OF THE 24-H MELATONIN PROFILE IN RESPONSE TO AFTERNOON DARK EXPOSURE

To investigate the adaptation of melatonin secretion to an abrupt time shift and the effects of sleep facilitation with a hypnotic, eight su bjects were submitted to an 8-h advance shift achieved by advancing be dtimes from 2300-0700 to 1500-2300. Each subject participated in two s tudies (i.e., placebo and zolpidem). Each study included a baseline pe riod with dim light during waking hours and 2300-0700 bedtimes in tota l darkness. Blood samples for determination of plasma melatonin were o btained at 20-min intervals for 68 h. Advanced exposure to sleep and d arkness resulted in a nearly 2-h advance of melatonin onset, which app eared within 6 h after Lights-out during the first shifted night, and an almost l-h advance of the melatonin offset. No further adaptation o ccurred during the second shifted sleep period. Zolpidem had no benefi cial effects on the adaptation of the melatonin profile. There was no relationship between sleep parameters and the magnitude of the melaton in shifts. Thus the overall advance of melatonin profiles was primaril y achieved during the initial exposure to an 8-h period of darkness. T he present data suggest that exposure to dark affects human circadian phase.