The emergence of salivary cortisol circadian rhythm and its relationship to sleep activity in preterm infants

OBJECTIVE The circadian rhythm of cortisol is established at between 8 and 12 postnatal weeks in term infants. However, there is limited information a bout the effect of prematurity on this rhythm. We evaluated the emergence o f the salivary cortisol circadian rhythm in premature infants and its relat ionship to the onset of sleep daily rhythm. DESIGN AND PATIENTS A longitudinal study of a group of nine premature infan ts (gestational age 31-34 weeks) was performed. Salivary samples were obtai ned in the morning and at night at 2, 4, 8, 12, 16, 20 and 24 postnatal wee ks and the babies' sleeping periods were recorded by their mothers. MEASUREMENTS Cortisol was determined by RIA in 25-mu l salivary samples. Tw o techniques based on assay coefficients of variation were used to characte rize the circadian pattern of cortisol. RESULTS Five infants (55%) established and maintained their cortisol rhythm at 2 and 8 postnatal weeks. In the remaining four infants the age of appea rance was 12 and 16 weeks. This rhythm emerged in the group as a whole betw een 8 and 12 postnatal weeks. The circadian rhythm of sleep was detected st arting from the eighth postnatal week. CONCLUSIONS Our data suggest that in this group of premature infants the ci rcadian maturation of the hypothalamic-pituitary-adrenal axis occurred at t he same postnatal age as reported for term infants and that there was a par allelism between the appearance of such rhythm and the onset of sleep rhyth m.