0770 Associations Between Biological Circadian Phase, Circadian Alignment, and Actigraphy-Derived Rest–Activity Rhythms in Adolescents

Abstract Introduction Adolescence brings biological and psychosocial changes that delay circadian timing and sleep–wake patterns. These delays conflict with early school schedules, contributing to circadian misalignment, short sleep, and daytime impairment. Although both biological and behavioral indices are used to assess circadian functioning, little is known about how biological circadian phase relates to actigraphy-derived rest–activity rhythm (RAR) features in adolescents. This study examined whether biological circadian phase (dim light melatonin onset, DLMO) or circadian alignment (DLMO-midsleep phase angle) were associated with RAR timing, amplitude, and stability. We also explored sex differences in these associations. Methods High school students (N=108; 54.6% female; M_age=17.4) completed eight days of actigraphy and two DLMO assessments (Thursday and Sunday) during the school year. Circadian alignment was defined as the phase angle between Thursday DLMO and actigraphy-based midsleep averaged across the prior three nights. RAR measures included timing indicators (acrophase, tRight, L5 onset), amplitude indicators (log amplitude, relative amplitude), and regularity indicators (interdaily stability, intradaily variability). Multiple regression models tested associations between three predictors—Thursday DLMO, Sunday DLMO, and weekday DLMO-midsleep—and each RAR parameter, controlling for sex. Sex-stratified models evaluated whether associations differed for males and females. Results In the full sample, later Thursday and Sunday DLMO were associated with later timing across all RAR timing indicators (βs ≈ .31–.52, ps < .001). Later Sunday DLMO was also associated with lower interdaily stability (β = –.19, p = .048). DLMO-midsleep was not associated with RAR measures overall. In sex-stratified models, however, a larger phase angle predicted earlier acrophase (β = –.28, p = .048) and earlier tRight (β = –.41, p = .003) among males, whereas among females, a larger phase angle was associated with lower relative amplitude (β = –.27, p = .04). Conclusion In this adolescent sample, biological circadian phase was most closely reflected by the RAR timing metrics, while a later circadian phase related to less stable activity patterns. Sex-specific associations suggest that circadian alignment manifests differently across sexes—showing earlier declines in daytime activity in males and weaker rhythm robustness in females. Support (if any) R01AA025626 (Hasler)