Toward an idiographic understanding of the role of sleep‐mood dynamics in adolescents' internalizing symptoms

Abstract Background Adolescence is marked by increased vulnerability to sleep disturbances and mood disorders. Understanding how day‐to‐day changes in sleep and mood are linked within the same individual is crucial for clarifying sleep's role in emerging internalizing disorders. However, the extent to which an adolescent's fluctuations in sleep predict next‐day mood may differ across individuals. This study examined such differences in person‐specific sleep‐mood dynamics and their links with depression and anxiety symptoms. Methods A total of 113 Swiss adolescents (57% female, mean age = 15.4 years, SD = 0.97) participated in three 3‐week waves of combined actigraphy, four daily Ecological Momentary Assessments (EMA), and weekly reports of depressive and anxiety symptoms. Bayesian multilevel models estimated person‐specific associations across two mood dimensions (sad‐vs‐happy and anxious‐vs‐calm) combined with sleep duration (SDur), sleep midpoint (SMid), and sleep regularity (SReg; i.e., overlap between each daily cycle and the participant's most typical cycle). Bayesian hypothesis tests examined evidence for differences in person‐specific associations and whether these differences predicted internalizing symptoms. Results Person‐specific associations involving SDur or SMid showed within‐person effects at the group‐level, but no convincing evidence for individual differences in this dynamic. In contrast, there were clear individual differences in how SReg related to anxious‐vs‐calm mood ( τ 2 = 0.47, [0.09; 0.79], BF 10 = 13.2). Adolescents whose calm mood was more strongly tied to a regular sleep–wake cycle reported fewer anxiety symptoms ( b = 3.57, 95%CrI [‐3.67; 9.31], BF 01 = 7.69). Conclusion Our findings suggest that adolescents whose daytime mood is closely coupled with regular sleep–wake patterns may experience protection against anxiety symptoms. They also show that adolescents differ meaningfully in these sleep–mood associations, challenging conventional group‐level approaches. Future work should extend this idiographic approach to more diverse populations and longer timescales to clarify the role of sleep–mood dynamics in internalizing disorders.