Long-term studies on immunoreactive human melatonin.

Immunoreactive melatonin has been studied in normal human volunteers, both men and women, at different times of day and different times of the year. At 2400, but not at 0800 or 1200, there are very large inter-individual variations in mean levels, the maximum difference being fivefold. Intra-individual variations are seen at different times of the year at 2400 and 0800, with low values in spring and autumn and high values in summer and winter.