Prolactin concentrations during the normal menstrual cycle: rhythms within rhythms.

Prolactin concentrations were measured in frequent blood samples collected over a 2 h period randomly distributed during day and night, during the follicular and the luteal phases of the normal menstrual cycle in women. Episodic fluctuations in prolactin concentrations were observed throughout the 24 h period and the pattern of these fluctuations appeared to vary depending on the time, and sleep wake state. Thus a changing pattern of the 'primary pulse' of prolactin may be responsible for the time related and sleep related changes in prolactin during the 24 h period.