Circadian rhythm maturation in infancy: a large-scale analysis of sleep-wake pattern development from 3 to 18 months...

Infant sleep-wake rhythms consolidate rapidly during the first year, yet normative trajectories and their variability remain poorly characterized. We examined sleep-wake development between 3 and 18 months using prospective, event-level data from a large international cohort.

Sleep logs from 175,593 infants (10.7 million logged days) were collected via a mobile tracking application. Parents recorded each sleep event prospectively. We analyzed developmental changes in daytime sleep and nap frequency, compensation during nap transitions, bedtime stability as a circadian marker, relationships between sleep timing and need, and rank-order stability of individual differences.

Total sleep decreased from 14.8 h at 3 months to 12.5 h at 18 months, reflected almost entirely by reductions in daytime sleep (226 to 101 min) as nap frequency declined from 3.9 to 1.0 per day. During the early nap transitions, the remaining naps lengthened to compensate for 88-89% of the sleep time lost when a nap was dropped. Compensation was lower during the transition from two naps to one nap (79%). Within-infant bedtime variability decreased most steeply before 9 months, consistent with circadian consolidation. Individual differences in bedtime regularity were highly stable across age periods (Spearman r = 0.73-0.77). Infants with greater sleep need had earlier bedtimes and more daytime sleep, while daytime sleep and time in bed were negatively correlated (r = -0.48).

The decline in total sleep across infancy results from reductions in daytime sleep through fewer and eventually shorter naps. Bedtime regularity increases most rapidly before 9 months, and individual differences emerge early and persist, suggesting enduring circadian traits are already established in infancy. Substantial inter-individual variability underscores the importance of individually tailored guidance.