THE HYPOTHERMIC EFFECT OF LATE EVENING MELATONIN DOES NOT BLOCK THE PHASE DELAY INDUCED BY CONCURRENT BRIGHT LIGHT IN HUMAN-SUBJECTS

This constant routine study (n = 9 men) compared the phase delay of th e circadian system induced by a single pulse of evening light (5000 Ix at 2100-2400 h) in the presence or absence of exogenous melatonin (5 mg p.o. at 2040 h). On the treatment day, light and melatonin protract ed and accelerated, respectively, the evening decline in core body tem perature (CBT). Subjective sleepiness ratings showed parallel shifts, the earlier the decline in CBT, the sleepier. On the post-treatment da y, light induced a phase delay in the mid-range crossing time of CBT d ecline independent of whether melatonin was co-administered or not. Su bjective sleepiness was delayed in parallel. The phase delay of the ci rcadian system by evening light appears to be independent of an immedi ate hyperthermic effect and is not mediated by melatonin. (C) 1997 Els evier Science ireland Ltd.