! Abstract An obstacle to the roll-out of evidence-based treatments (EBTs) for severe mental illness (SMI) is that the context for the implementation typically differs from the original testing context causing a lack of “fit” between the setting and the EBT. We propose to evaluate if adapting a specific treatment to improve the contextual fit improves outcomes in a setting that typifies this challenge—community mental health centers (CMHCs). Following the Experimental Therapeutics Approach, the target is sleep and circadian dysfunction. In SMI, sleep and circadian dysfunction undermines affect regulation, cognitive function and physical health, predicts the onset and worsening of symptoms and is often chronic even with evidence-based SMI treatment. Prior treatment studies have been disorder-focused—they have treated a specific sleep problem (e.g., insomnia) in a specific diagnostic group (e.g., depression). However, real life sleep and circadian problems are not so neatly categorized, particularly in SMI. Hence, we developed the Transdiagnostic Intervention for Sleep and Circadian Dysfunction (TranS-C) to treat a wide range of sleep and circadian problems experienced in SMI. With NIMH support, including a study in CMHCs, we established that TranS-C engages the target. Yet gaps remain: 1) Thus far, the TranS-C providers have been employed, trained and supervised by the university. We will determine if TranS-C can be effectively delivered by providers within CMHCs. 2) We will test a version of TranS-C that has been adapted to improve the fit and to address potential barriers to scaling TranS-C. The rigorous adaptation process used theory, data and stakeholder inputs. 3) We will study ad hoc adaptations made by providers to TranS-C. 4) We include two stages; namely, the Implementation Phase (2 years) and the Sustainment Phase (1 year). The latter responds to urgent calls to study the sustainability of EBTs. Guided by the Replicating Effective Programs (REP) framework, in this Hybrid Type 1, 4-year study, 8 CMHC clinic sites will be cluster randomized to either Standard or Adapted TranS-C. Then, within each CMHC site, patients will be randomized to immediate TranS-C or to Usual Care followed by Delayed Treatment (UC-DT). A total of 96 providers and 576 patients will participate. Patients will be assessed pre, mid and post-treatment and at 6 months follow-up. UC Berkeley will co-ordinate the research, facilitate implementation, collect data etc. Providers within an established network of CMHCs will implement TranS-C. SA1 is to confirm that both Standard vs. Adapted TranS-C, compared to UC-DT, improve sleep and circadian functioning and reduce functional impairment and disorder-focused psychiatric symptoms. SA2 is to evaluate the fit, to the CMHC context, of Standard vs. Adapted TranS-C. SA3 will examine if better fit mediates the relationship between treatment condition and patient outcome. This research will determine if sleep and circadian problems ...