Thermoregulatory and soporific effects of very low dose melatonin injection

The effect of a rapid increase in circulating melatonin on body temperature s and sleepiness was investigated in eight young adults at 1000. Melatonin administered intravenously at 10- and 30-mu g doses, but not 3 mu g, result ed in elevated plasma and saliva levels consistent with endogenous levels m easured in adults at night. Melatonin at 10 and 30 mu g significantly atten uated the daytime increase in rectal core temperature (P < 0.05 for both). The mean maximum rectal core temperature differences between saline and mel atonin treatment were 0.11 +/- 0.03 degrees C, 0.16 +/- 0.04 degrees C, and 0.18 +/- 0.04 degrees C after the 3-, 10-, and 30-mu g melatonin doses, re spectively. All three doses significantly increased hand temperature compar ed with saline (P < 0.05) within 30 min. The mean maximum hand temperature differences were 0.72 +/- 0.12 degrees C (3 mu g), 0.95 +/- 0.15 degrees C (10 mu g), and 0.65 +/- 0.11 degrees C (30 mu g). Foot temperature and subj ective sleepiness measures did not change at any melatonin dose. The result s suggest that daytime intravenous injection of melatonin to achieve normal nocturnal levels in young adults may produce significant thermoregulatory changes without soporific effects.