Suicide is the second leading cause of death among 15-24-year-olds. To address this significant public health concern, this study seeks to advance our understanding of the daily interplay of sleep and the biological clock and how teens think of themselves with their thoughts of suicide. This proposal's central hypothesis is that a prolonged time interval between feeling sleepy and actual bedtime creates unique opportunities to engage in nighttime self-critical rumination-repetitive negative thinking about self and diminishes teens' ability to self-reassure (supportive of self). We also investigate how neural processing of self-criticism and self-reassurance varies based on the circadian timing, potentially affecting their subsequent sleep and next-day suicidal thoughts. Our central methodology is to examine the interface of sleep and biological clock and nighttime self-critical rumination and self-reassurance with next-day suicidal thoughts using ecological momentary assessment (EMA}, gold-standard assessments of circadian timing and sleep, and functional magnetic resonance imaging (fMRI) in 60, 13-17-year-old youth. Teens will be recruited on a range of suicidal thoughts from the inpatient, partial hospitalization, and outpatient care, as well as the community. This age group represents a critical developmental period for self-referential processes and sleep changes, and youth in this age group experienced a sharp increase in suicide rates over the last decade. This proposal innovatively links nighttime self-critical rumination and self-reassurance with next-day subsequent suicidal thinking via circadian rhythms and sleep by integrating (i) state-of-the-art biological clock and sleep measures for objectivity; (ii) EMA techniques for real-life meaning; and (iii) fMRI techniques to identify neural treatment targets. This research is significant because characterizing brain-behavior mechanisms of the association of circadian timing and sleep behavior with nighttime self-critical rumination, self-reassurance, and next-day suicidal thinking can contribute to developing novel mobile interventions that target self-referential thinking and sleep to reduce suicide risk in youth.