Development of circadian rest-activity rhythms during the first year of life in a racially diverse cohort

Abstract Study Objectives To describe the development of circadian rest-activity rhythms (CRARs) during infancy in a racially diverse cohort. Methods We studied 414 infants from the Nurture birth cohort (51.2% female, 65.2% Black) who wore actigraphs on their left ankles for 4 days and nights at 3, 6, 9, and 12 months. We quantified CRARs using cosinor and non-parametric circadian rhythm analysis, and investigated change in CRARs over time, comparing 6, 9, and 12 months to CRARs at 3 months. We adjusted for baseline and time-varying covariates and used function-on-scalar regression (FOSR) to identify the specific times of day at which activity changes occurred. Results Across the first year, daily mean and peak activity levels and day-to-day activity level regularity increased, and activity level fragmentation and nighttime activity decreased. Only at 9 months, compared to at 3 months, did timing of peak activity levels and the most active periods shift later, while timing of least active periods shifted earlier. FOSR analyses showed that mean activity levels decreased during nighttime hours and increased during daytime hours, with the most pronounced changes at 9 months. Conclusions Among racially diverse infants, CRARs became more robust, stable, and less fragmented over time. Findings suggest the greatest change from 3-month CRARs occurs at 9 months, which may be a key period of CRAR development. This and future research will contribute to our understanding of normative infant CRAR development in diverse populations and enable us to identify infants who may benefit from intervention.