MELATONIN-INDUCED TEMPERATURE SUPPRESSION AND ITS ACUTE PHASE-SHIFTING EFFECTS CORRELATE IN A DOSE-DEPENDENT MANNER IN HUMANS

Melatonin is able to phase-shift the endogenous circadian clock and can induce acute temperature suppression. It is possible that there is a direct relationship between these phenomena. In a double-blind, placebo-controlled crossover study, 6 healthy volunteers maintained a regular sleep/wake cycle in a normal environment. From dusk until 24:00 h on days (D) 1-4 subjects remained in dim artificial lighting(< 50 lux) and darkness(< 1 lux) from 24:00-08:00 h. At 17:00 h on D3 either melatonin (0.05 mg, 0.5 mg or 5 mg) or placebo was administered. Melatonin treatment induced acute, dose-dependent temperature suppression and decrements in alertness and performance efficiency. On the night of D3, earlier sleep onset, offset and better sleep quality were associated with increasing doses of melatonin. The following day, a significant dose-dependent phase-advance in the plasma melatonin onset time and temperature nadir (D4-5) was observed with a trend for the alertness rhythm to phase-advance. A significant dose-response relationship existed between the dose of oral melatonin, the magnitude of temperature suppression and the degree of advance phase shift in the endogenous melatonin and temperature rhythms, suggesting that acute changes in body temperature by melatonin may be a primary event in phase-shifting mechanisms.