Charting infant sleep cycle development using actigraphy: longitudinal evidence for ultradian cycle lengthening within the first year of life from 35,000 hours of sleep

Sleep is marked by ultradian cycles coinciding with the alternation of rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. These sleep cycles are a striking feature exhibited from infancy through adulthood, yet their underlying mechanisms and functional relevance remain elusive, calling for large-scale longitudinal studies. Here we leverage Locomotor Inactivity During Sleep (LIDS), an accessible marker of sleep cycles in adults, to chart ultradian sleep cycle dynamics in infants at scale. Specifically, we analyzed $>35,000$ hours of sleep from a longitudinal dataset of $152$ infants with actigraphy at 3, 6, and 12 months of age. Using complementary signal processing techniques, we demonstrate the existence of rhythmic patterns in infant LIDS with cycle lengths of ~ 60 minutes. Cycles were shorter in infants than parents (62 min 95%CI $[56,67]$ vs. 81 min $[74,88]$) and increased by ~ 10 min from $3$ to $12$ months, mirroring previous results in smaller samples for NREM-REM sleep cycles. This increase was partially mediated by increasing sleep bout duration ($1.0$ min/h $[0.9,1.2]$). Longer cycles were also found in infants still breastfed at $12$ months ($+2.5$ min $[0.4,4.5]$) and their breastfeeding mothers ($+6.7$ min $[0.5,12.9]$). Furthermore, inactivity was lower at sleep onset, declined more rapidly and showed a greater amplitude in infants than parents, but all three parameters began to mature already within the first year. Overall, our results support a link between patterns in limb inactivity and sleep cycle physiology in infancy, underscoring the potential of studying these in large-scale cohorts for developmental and health outcomes.