Ontogeny of Circadian Rhythmicity for Melatonin, Serotonin, and N‐Acetylserotonin in Humans

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Abstract
The serum concentration of melatonin, serotonin, and N-acetylserotonin were measured by RIA procedures in 28 infants aged 1 week to 9 months. Blood specimens were obtained at 1200 hr and 2400 hr. A day-night difference in serum serotonin was present immediately after birth. A significant (P < 0.001) decrease in serum serotonin concentrations at 1200 hr and 2400 hr was observed from the first month of age to the third to ninth month of age. A significant (P < 0.05) difference in day-night N-acteylserotonin concentration is first seen at age 1–3 months. Serum melatonin concentrations, though detectable, did not show any day-night difference at birth. Melatonin concentrations progressively increased up to the third month of age, and a significant (P < 0.01) day-night difference appeared thereafter. The results indicate that in humans the circadian organizatijon for serotonin already exists at birth, and the circadian melatonin rhythm develops after birth.