Melatonin-induced decrease of body temperature in women: a threshold event.

Whether the biological effect of melatonin in humans is directly related to the circulating levels of the hormone, has not heretofore been investigated. In this study, we investigated whether previously described hypothermic melatonin properties are dose related. The nocturnal decline of the body temperature (BT) observed in 16 early follicular phase women, following placebo administration at 18.00 h, was compared with that observed during the preceding or following night, after melatonin suppression with the beta 1-adrenergic antagonist atenolol (100 mg). In 6 subjects (37.5\%) with lower nocturnal melatonin levels (p < 0.05) atenolol induced a complete melatonin suppression and an attenuation of the nocturnal BT decline (p < 0.02), whereas in the remaining 10 subjects (62.5\%) atenolol induced an incomplete melatonin suppression with no modification of the nocturnal BT decline. During a 3rd night, 2 of the 6 subjects with complete and 6 of the 10 subjects with incomplete melatonin suppression blindly received atenolol plus melatonin (1 mg at 19.30 h and 0.75 mg at 21.00 and 23.00 h). Exogenous melatonin restored the full expression of the nocturnal BT decline in the 2 subjects with complete melatonin suppression, but did not modify the BT decline in the 6 subjects with atenolol-induced incomplete melatonin suppression. Our data show that markedly, but not completely attenuated nocturnal melatonin levels are sufficient to exert maximal thermoregulatory effects, indicating rather a threshold than a dose-response effect of melatonin action on human BT.