PROJECT SUMMARY/ABSTRACT Suicide is a leading cause of death among adolescents in the US, with rates of suicidal ideation (SI) and behavior continuing to increase in recent years. Given the simultaneous increase in social media use (SMU), there is public concern about the relationship between SMU and SI. Methodological limitations in prior research limit our understanding of this relationship and the development of empirically-based recommendations for best practices with SMU. Most studies: 1) only focus on SMU patterns (duration/frequency), 2) assess SMU via self-report methods at a single timepoint, 3) use cross-sectional designs limiting directionality, and 4) do not consider influences of other media or in-person social experiences. We also do not yet understand the potential mechanisms linking SMU and SI. This makes it challenging to identify the nuanced, objective, temporal, and unique relation between SMU and SI, or potential mechanisms. The proposed study will address these limitations using rigorous, innovative, and youth-centered approaches, including ongoing consultation of the proposed study with our youth advisory board. We will use intensive monitoring approaches to examine the relationships between three SMU domains in relation to SI among adolescents recently discharged from psychiatric hospitalization for suicide-related risk. SMU patterns (duration/frequency) will be objectively captured using smartphone sensing methods. Negative SMU (i.e., social rejection via social comparison, negative interactions), positive SMU (i.e., social connection via social support, positive interactions), and SI will be assessed using ecological momentary assessment (EMA). This approach allows us to examine directional and proximal relations between SMU and SI (Aim 1), and to identify SMU’s unique effects compared to other screentime and in-person social experiences. This study also will evaluate sleep disruption as a potential mechanism linking SMU and SI. Sleep disruption (e.g., shorter duration, poorer quality) is a proximal risk factor for SI and modifiable target for prevention. While SMU has been linked to sleep disruption, sleep disruption also may influence SMU patterns and experiences to heighten risk for SI. This study will capture sleep using actigraphy and sleep diaries to examine the bidirectional relationships between SMU (patterns, negative, positive) and sleep disruption (Aim 2a), and in risk for next-day SI (Aim 2b). Adolescents (N=85, 13-17 years) in the first 3 months after psychiatric hospitalization will be recruited and participate in an intensive monitoring study over 28 days, leveraging the infrastructure from an ongoing multi-site R01. Post-hospitalization is a high-risk period for suicide, and these high-risk youth may be more affected by SMU, making this period critical for suicide prevention. Results from this innovative, multi-method study will advance our understanding on relationships between SMU, sleep, and SI among yo...