Project Summary Adolescent suicide rates have increased over the past two decades, with the recent COVID-19 exacerbating pre-pandemic risk factors including increases in online social messaging. We propose that online social messaging for adolescents serves as a “social zeitgeber” (an environmental “time giver” that synchronizes diurnal rhythm) and that late night online social messaging contributes to delays in sleep onset. Over time, delays in sleep onset in combination with inability to phase shift the wake time for adolescents, contributes to impairments in cognitive and emotional capacity for managing daily stress, resulting in overall vulnerability throughout the day as well as a shift of vulnerability to the evenings when research has shown increases in adolescent suicidal thoughts and behaviors. The present research longitudinal research will examine these processes in the real world using a combination of (1) text extraction of online social messaging during the intensive sampling period, (2) ecological momentary assessment (EMA), and (3) wearable technology in combination with predictive algorithms. Findings will inform interventions that can range from ecological momentary interventions to adaptations to interventions such as Interpersonal and Social Rhythm Therapy.