MODIFICATION OF CIRCADIAN BODY-TEMPERATURE RHYTHM DURING THE LUTEAL MENSTRUAL PHASE - ROLE OF MELATONIN

In women during early follicular phase (FP), the rise of melatonin at night accounts for 40% of the nocturnal core body temperature (T-c) de cline. In seven normal-cycling women, the circadian rhythms of T-c and melatonin of the FP were compared with those of the luteal phase (LP) . In addition, in both phases the T-c response to daytime melatonin ad ministration was investigated. Melatonin levels were comparable during the two menstrual phases, but the nocturnal melatonin onset was delay ed by 90 min in the LP (P < 0.01). This was accompanied by a delay of the nadir of the T-c circadian rhythm (P < 0.002), a 0.3 degrees C ele vation (P < 0.005) of the mean 24-h value, and a 40% blunting (P < 0.0 02) of the amplitude. This attenuation of circadian T-c in LP women wa s replicated in two estrogen-treated hypogonadal women by the administ ration of medroxyprogesterone acetate. The daytime administration of m elatonin (2.5 mg) decreased T-c during the FP (P < 0.01) but was ineff ective in the LP Present data indicate that in LP, in association with high progesterone levels, an attenuated and phase-delayed circadian T -c rhythm may, in part, be due to a reduced effect of melatonin.