The circadian rhythm of plasma melatonin during the normal menstrual cycle and in amenorrheic women.

Plasma melatonin, PRL, and LH levels were measured in samples collected every 2 h for 24 h from 14 normally cycling women during the early follicular, periovulatory, and luteal phases of their menstrual cycles. Plasma melatonin levels also were measured in samples collected at the same interval from 7 patients with hypothalamic amenorrhea. A distinct daily rhythm in plasma melatonin was evident in all subjects, with peaks occurring around 0300 h. Each woman's rhythm was remarkably consistent throughout the menstrual cycle (in terms of the phase, amplitude, and total melatonin secreted). Plasma PRL levels also exhibited daily rhythms which did not change during the menstrual cycle; the nocturnal peak plasma PRL level tended to occur 1-2 h after that for melatonin. Among the amenorrheic women, both daytime and nighttime melatonin levels were significantly higher (P less than 0.005) than in the normal women. Their plasma PRL levels were similar to those in the normal women. We conclude that, as for PRL, the circadian rhythm of melatonin secretion does not change significantly during the normal menstrual cycle. The elevated plasma melatonin levels in women with hypothalamic amenorrhea suggest that the hormone may be involved in the neuroendocrine pathology underlying this disorder.