Charting infant sleep cycle development using actigraphy: longitudinal evidence for ultradian cycle lengthening within the first year of life from 35 000 h 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 $> \text{35\,000}$ h of sleep from a longitudinal dataset of $\text{152}$ 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 approximately $\text{60}$ min. Cycles were shorter in infants than parents (62 min 95%CI $[\text{56, 67}]$ vs. 81 min $[\text{74,\ 88}]$) and increased by approximately 10 min from $\text{3}$ to $\text{12}$ months, mirroring previous results in smaller samples for NREM-REM sleep cycles. This increase was partially mediated by increasing sleep bout duration ($\text{1.0}$ min/h $[\text{0.9, 1.2}]$). Longer cycles were also found in infants still breastfed at $\text{12}$ months ($\text{2.5}$ min $[\text{0.4, 4.5}]$) and their breastfeeding mothers ($\text{6.7}$ min $[\text{0.5, 12.9}]$). 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. Statement of Significance Infancy is a time of profound change in sleep, spurring theories about underlying drivers, and links with developmental outcomes. Studying sleep repeatedly at this early age, however, is difficult, but wearables may help to overcome the data shortage. Here, we used easy-to-record limb movement from ankle actigraphs as a window into infant sleep physiology, going beyond its established use in sleep-wake monitoring. We demonstrate the presence of 60-min cycles in movement during sleep compatible with the pioneering discoveries of alternating rapid eye movement (REM) and non-REM (NREM) sleep in infants. The gradual lengthening of these cycles in our large sample underscores previous evidence of NREM-REM cycle lengthening during infancy, paving the way for studies linking cycle dynamics with developmental trajectories in large birth cohorts.