PROJECT SUMMARY/ABSTRACT Suicide is the 2nd leading cause of death among 10-14-year-olds and the 3rd leading cause of death among 15-24 year-olds in the United States. Reducing adolescent suicide rates is an urgent public health objective. Insomnia is a robust yet neglected transdiagnostic risk factor for suicidality and suicide deaths in adolescents that is modifiable using brief, evidence-based behavioral treatments. Despite this, to the best of our knowledge, insomnia treatments have never been studied in combination with suicide-focused treatments in youths. The purpose of the proposed research is to conduct an initial feasibility and preliminary effectiveness trial of combining evidence-based behavioral treatments for insomnia and suicidality/self-harm as a novel approach to adolescent suicide prevention. We anticipate that augmenting a suicide-focused treatment with an insomnia treatment will have a synergistic effect and lead to greater reductions in self-harm (SH) behaviors and suicidal ideation (SI) than suicide-focused treatment alone. We will use dialectical behavior therapy (DBT) as the suicide-focused treatment for the present study and internet-delivered, digital cognitive-behavioral therapy (dCBTI) for insomnia as the sleep-focused intervention. We will begin with a small open trial to obtain input from five youth and their DBT therapists to inform the proposed feasibility trial, adjusting the protocol as needed to enhance youth engagement in dCBTI. We will then use the modified protocol to conduct a pilot RCT with 40 youth, ages 12-18, with insomnia and high suicide risk to 6 months dCBTI plus DBT or to DBT alone. We will assess feasibility, acceptability, and safety of dCBTI + DBT, as well as reductions in insomnia symptom severity, SH, and SI. Following a baseline assessment, insomnia, SI and SH outcomes will be measured every 4 weeks over the 24-week treatment period to allow us to examine time to remission as well as to increase statistical power and mitigate impact of attrition. Exploratory analyses will examine if reduction in insomnia symptom severity mediates any observed between-condition differences in SH and SI. To the best of our knowledge, this will be the first study to explore the effectiveness of combining both an evidence-based insomnia treatment and a suicide-focused treatment for adolescents. We believe the approach of combining two existing evidence-based treatments that target risk factors for suicide is innovative and novel, as well as practical. This approach is consistent with the 2022 NIMH Strategic Plan for Research, Objective 3.2: Develop strategies for tailoring existing interventions to optimize outcomes. By leveraging existing treatments that we already know work and have robust uptake in the real world, we reduce time spent on treatment development and can move faster to dissemination, should the approach prove effective. If dCBTI + DBT is effective, our goal will be to disseminate dCBTI broadly as a...