Effects of a four-day nocturnal melatonin treatment on the 24 h plasma melatonin, cortisol and prolactin profiles in humans.

An oral preparation of melatonin was administered daily at 22.00 h to 6 healthy volunteers during summer on 4 consecutive days (days 1-4). The daily dose was 8 mg of melatonin as a single. Three 24-h melatonin, cortisol and prolactin profiles were determined in plasma by radioimmunological methods: 1) before treatment (day 0); 2) the first day after the 4-day treatment had been stopped (day 5), 3) the third day after withdrawal of this treatment (day 7). For the melatonin rhythm, an advanced phase was observed at day 7 vs day 0, whereas the amplitude and the mesor were not modified, whatever the day. For the prolactin profile, a significant increase as compared with the control day (day 0) was detected only at day 7 between 19.00 and 21.00 h. No modification was recorded for the plasma cortisol secretion. These results suggest that melatonin, when administered at a high dose over a short period, can influence the endocrine rhythms, and especially its own endogenous secretion. This effect must be investigated over several days after the treatment has ended.