Project Summary/Abstract Suicide is the second leading cause of death among 10-34 year-olds. To address this significant public health concern, this award seeks to advance what is known about the interplay of self-critical rumination ─ defined as persistent negative-self-evaluation ─ and self-reassurance ─ providing compassion to self with sleep behavior and circadian timing in the context of youth suicide risk. This award's central hypothesis is that maturational changes in circadian timing and homeostatic pressure, resulting in increased time interval between biological sleep clock and actual bedtime and reduced sleep time, could create unique opportunities to engage in self- critical rumination at night and be linked to decreased ability to use self-reassurance during the day. Our central methodology is to examine the interface between self-critical rumination and self-reassurance with sleep and circadian timing and suicidal thoughts and behavior (STB) using ecological momentary assessment (EMA), gold-standard assessments of circadian timing and sleep behavior, and task-dependent and task- independent functional magnetic resonance imaging (fMRI) in 100, 10-15 year-old children recruited on a range of STB from inpatient, outpatient, and community settings. This age group represents a critical developmental period for these self-referential processes and youth in this age-group experienced a sharp increase in suicide rates over the last decade. Our research goals are to: (i) test associations of circadian timing and sleep behavior with self-critical rumination and STB; (ii) define the association between sleep duration, self-reassurance, and STB (iii) identify fundamental neural correlates of self-critical rumination and self-reassurance via task-dependent and resting state fMRI. This award is significant because characterizing brain-behavior mechanisms of the association of circadian timing and sleep behavior with self-critical rumination, self-reassurance, and STB can contribute to development of novel, mechanistically- informed interventions to reduce suicide risk in youth. This proposal's innovation is in linking circadian timing and sleep behavior to severity and time-of-day for the use SCritR and SReas and subsequent STB by integrating (i) state-of-the-art circadian timing and sleep measures; (ii) EMA techniques for real-life meaning; (iii) task- dependent and task-independent fMRI techniques; and (iv) focus on a trans-diagnostic sample of youth selected on the range of STB vs. a single psychiatric disorder. The findings from this proposal will inform future applications for funding that aim to ultimately develop mechanistically-informed, just-in-time interventions.