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) decline. 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 administration was investigated. Melatonin levels were comparable during the two menstrual phases, but the nocturnal melatonin onset was delayed 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 elevation (P < 0.005) of the mean 24-h value, and a 40% blunting (P < 0.002) of the amplitude. This attenuation of circadian T-c in LP women was replicated in two estrogen-treated hypogonadal women by the administration of medroxyprogesterone acetate. The daytime administration of melatonin (2.5 mg) decreased T-c during the FP (P < 0.01) but was ineffective 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.