A longitudinal study of basal cortisol in infants: Intra-individual variability, circadian rhythm and developmental trends

Mothers with normally developing babies were visited in their homes during 13 consecutive weeks, when the babies were around 5–8 months of age. Basal salival cortisol measures were taken for both the baby and the mother on arrival.The infants’ basal cortisol decreased linearly with age, was negatively related to sleep, and did not show adult-like circadian declines from morning to mid-afternoon. Furthermore, while the infants showed relative stability across individuals, they displayed great intra-individual variability across assessments. Contrarily, the mothers displayed important inter-individual variability, together with a relative stability across assessments. The infants’ important intra-individual variability was not affected by gender, nor time of visit, nor was it related to the mothers’ basal cortisol. Daily measures of basal cortisol taken in a subgroup of infants indicated the day-to-day intra-individual variability to be of the same magnitude as the week-to-week variability.The question of how the intra-individual variability in basal cortisol affects assessments of cortisol in infancy is addressed. The aggregation of data with the goal of increasing the reliability of the assessments is shown to be inadequate for infant basal cortisol.