PROJECT SUMMARY/ABSTRACT Insomnia disorder is a common and consequential mental health problem, with prevalence estimates at approximately 10%. The American College of Physicians recommends cognitive behavioral therapy for insomnia (CBT-I), an efficacious “mind” (psychological and behavioral) treatment, as the first-line treatment for adults with insomnia. Yet, about half of individuals experience residual insomnia symptoms after CBT-I. Accordingly, researchers have sought to augment CBT-I, primarily with pharmacotherapy, but many people do not want to use pharmacologic agents due to concerns about side effects and dependence. Passive-body heating (PBH), a “body” treatment, involves heating the body via hot baths or showers, infrared sauna, or other heat sources. PBH has been found to reduce the time needed to fall asleep and to improve sleep quality. PBH may improve sleep by increasing skin temperature and decreasing core body temperature, a dynamic associated with sleep onset. CBT-I and PBH thus target distinct factors that may contribute to insomnia, and their integration holds promise as a multi-component treatment for insomnia disorder. The overarching goal of the proposed project is to conduct feasibility and acceptability testing of an integrated CBT-I and PBH intervention for insomnia disorder. Participants will complete digital CBT-I (dCBT-I), which is similarly efficacious to traditional CBT-I but with increased potential for scalability, and PBH sessions using a water-sparing, commercially available, lightweight infrared sauna blanket that requires only a regular household outlet. Study 1 will iteratively refine and protocolize study procedures, such as study assessments and the frequency and duration of PBH sessions, in a single-arm trial (N=10). Study 2 will test recruitment, randomization, and retention procedures in a pilot two-arm RCT comparing dCBT-I+PBH to dCBT-I alone (N=40). Studies 1 and 2 will both include a 9-week intervention period and a 6-month follow-up period. The proposed project will prepare the integrated intervention and RCT procedures for a future remotely delivered clinical efficacy trial that tests for added benefit of dCBT-I+PBH above dCBT-I alone. The proposed mind-body treatment option for insomnia holds potential as a scalable, cost-effective, remotely delivered, non-drug treatment option for millions of individuals struggling with insomnia.