Project Summary Deaths from suicide continue to be a major public health concern, particularly among youth and young adults, for whom suicide is the second leading cause of death. To advance suicide prevention, research that examines interacting processes covarying with and preceding acute suicidal behaviors are needed. Non-suicidal self-injury is identified as a robust predictor of suicidal behavior, with theories suggesting it confers risk by contributing to decreased aversion to, and increased cognitive bias towards, self- harm. Very little is known about the short-term temporal course of NSSI in relation to suicidal behavior, and information about possible mechanisms that facilitate, or reduce, transition to suicidal behavior among those engaging in NSSI is sparse. The proposed project aims to provide data filling gaps in the field of suicide research by conducting a micro-longitudinal study of transdiagnostic processes believed to interact with and influence risk for suicidal behavior conferred by NSSI. Using the Integrative-Motivational-Volitional (IMV) theory of suicide as a guide, the current study proposes that variability in sleep quality and emotional reactivity will interact with self-perceptions and perceived social connectedness to influence NSSI features and mechanisms of risk to predict near-term, acute, change in suicidal behavior. Young adults reporting current NSSI and suicidal ideation will wear actigraph watches (to obtain sleep data) and complete daily diary assessments of the study variables along with weekly lab-based assessment for 4 weeks, and complete 1- and 2-month follow-ups. Multi- level modeling analyses will examine the moderating effects of sleep and emotional reactivity on the proposed risk-processes to influence NSSI and suicidal behaviors over days, weeks, and months. The results will provide novel and essential information to the field of suicidology by advancing current knowledge about acute risk for suicide and have potential to significantly impact clinical practice by identifying specific, modifiable markers of risk that can translate into clinical targets for assessment and intervention strategies. Relevance Suicide rates continue to rise in the U.S. and research examining psychological processes impacting acute transitions to suicidal behavior is desperately needed. Identifying mechanisms that influence other factors to increase or reduce near-term risk for suicide can advance both the science and prevention of suicide.