The influence of exogenous estrogen on the circadian periodicity of circulating prolactin in women.

Ethinylestradiol (400 pg/day) does not only stimulate prolactin release in normal cycling women but also modifies the pattern of the circadian periodicity of circulating prolactin: the nocurnal rise is of reduced amplitude but covers a larger part of the nyctohemeral period as compared to the situation during a control period in the same subjects. The values start rising long before sleep, i.e., -t 2 pm, under estrogen treatment and during sleep, i.e., at 4 am, during the control period. In both cases, however, the values remain high until 8 am and are minimal at 12 am.