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.