0254 Longitudinal Patterns of Circadian Phase and Sleep Timing Across Early Childhood

Abstract Introduction Early childhood represents a period of profound developmental changes in brain maturation and sleep consolidation. However, little is known about the developmental trajectory of the circadian system and its relationship with sleep timing during this period. Longitudinal data on circadian phase across childhood are rare, limiting our understanding of individual differences and stability throughout development. Describing these patterns is essential to identify factors contributing to behavioral sleep problems. We examined longitudinal changes in circadian phase and its relationship with sleep timing across early childhood. Methods Data were collected from healthy children between 2007 and 2016. Circadian phase and sleep timing were assessed across four ages: 2-3 years (T1, N=58), 3-4 years (T2, N=49), 4-5 years (T3, N=32), and 5-6 years (T4, N=35). Children maintained their habitual sleep schedule for 5 days, confirmed via actigraphy. They then completed an in-home assessment of their dim light melatonin onset (DLMO). Linear mixed-effects models tested whether DLMO, sleep timing (bedtime, sleep onset, midsleep time, sleep offset), or phase angles of entrainment varied across timepoints. Results Average DLMO was 19:30 ± 0:47 at T1, 19:30 ± 0:59 at T2, 19:16 ± 1:06 at T3, and 19:21 ± 1:10 at T4. DLMO preceded bedtime by an average of 36 ± 47 min at T1, 43 ± 60 min at T2, 52 ± 63 min at T3, and 36 ± 48 min at T4. There were no significant effects of timepoint on DLMO, sleep timing, or phase angles. Across all timepoints, children with later DLMOs had significantly later sleep onsets and midsleep times. Later DLMOs were associated with significantly later bedtimes at T1 (r=0.43), T2 (r=0.30), and T3 (r=0.45), but not at T4 (r=0.27). Conclusion Both circadian phase and its relationship with sleep timing appear remarkably stable across ages 2-6 years, even as children drop daytime naps and begin formal schooling. These foundational data on early circadian physiology can help guide the development of targeted interventions for children with behavioral sleep problems, supporting better alignment between circadian rhythms and sleep timing. Support (if any) Data collection supported by K01-MH07643 & R01-MH08566 awarded to the late Dr. Monique LeBourgeois.