GENDER DIFFERENCES IN THE TEMPORAL ORGANIZATION OF PROLACTIN (PRL) SECRETION - EVIDENCE FOR A SLEEP-INDEPENDENT CIRCADIAN-RHYTHM OF CIRCULATING PRL LEVELS - A CLINICAL RESEARCH-CENTER STUDY

Although a nocturnal rise in PRL secretion is well known, it has long been presumed to be evoked by sleep. To determine whether PRL secretio n was driven by a sleep-independent circadian rhythm, we studied 12 me n and 10 women using a constant routine protocol. Under the constant r outine conditions of continuous semirecumbent wakefulness in constant indoor room light with hourly meals distributed throughout the day and night, a persistent circadian rhythm of PRL secretion was present in men and in women at the follicular and luteal phases of the menstrual cycle. Furthermore, the amplitude of this rhythm in women was signific antly greater than that in men. The present data demonstrate the prese nce of a robust sleep-independent endogenous circadian rhythm of PRL s ecretion in humans. We hypothesize that this endogenous component of t he circadian rhythm of PRL secretion together with those of body tempe rature, urine production, and cortisol, TSH, and melatonin secretion a re driven by the central circadian pacemaker located in the suprachias matic nucleus of the hypothalamus.