Maturation of the Circadian Rhythm of the Adrenocortical Functions in Human Neonates and Infants

The cortisol levels in the peripheral blood were measured radioimmunologically at 08.00, 12.00,. 16.00, 20.00, 24.00 and 04.00 h in human neonates and infants with ages of 1, 2 and 3 days, 1, 2 and 4 weeks, 2, 3 and 6 months and 1 and 3 years. During the first 2 days of extrauterine life the neonates had high plasma cortisol levels without a circadian rhythm. During the first 2 months there were decreased cortisol levels in the infants, but the diurnal rhythms were still absent, and free-running fluctuations were observed. The typical circadian rhythms of the plasma cortisol levels were present in infants aged 3 months; these rhythms were identical with the diurnal changes of 1- and 3-year-old infants. In the groups of patients studied the data suggest that the circadian rhythm of the adrenocortical function may develop to the adult-type pattern already during early infancy.