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

The circadian rhythm of cortisol is established at between 8 and 12 postnatal weeks in term infants. However, there is limited information about the effect of prematurity on this rhythm. We evaluated the emergence of the salivary cortisol circadian rhythm in premature infants and its relationship to the onset of sleep daily rhythm. A longitudinal study of a group of nine premature infants (gestational age 31-34 weeks) was performed. Salivary samples were obtained in the morning and at night at 2, 4, 8, 12, 16, 20 and 24 postnatal weeks and the babies' sleeping periods were recorded by their mothers. Cortisol was determined by RIA in 25-microl salivary samples. Two techniques based on assay coefficients of variation were used to characterize the circadian pattern of cortisol. Five infants (55%) established and maintained their cortisol rhythm at 2 and 8 postnatal weeks. In the remaining four infants the age of appearance was 12 and 16 weeks. This rhythm emerged in the group as a whole between 8 and 12 postnatal weeks. The circadian rhythm of sleep was detected starting from the eighth postnatal week. Our data suggest that in this group of premature infants the circadian maturation of the hypothalamic-pituitary-adrenal axis occurred at the same postnatal age as reported for term infants and that there was a parallelism between the appearance of such rhythm and the onset of sleep rhythm.