Consecutive melatonin circadian rhythms in normal volunteers.

Circadian plasma melatonin secretion patterns were determined in 12 healthy men over 3 consecutive 24-hour periods. The rhythms showed marked intra-individual stability, but pronounced interindividual variation. Plasma melatonin may serve as a reliable biological marker. Plasma melatonin assayed between 23h00 and 01h00 gave a reliable indication of an individual's circadian melatonin rhythm.