1 Abstract The goal of this research is to contribute to knowledge on the implementation and sustainment of an evidence-based treatment for severe mental illness (SMI) in a network of 8 community mental health centers (CMHCs) in California. The evidence-based treatment for this study is the Transdiagnostic Sleep and Circadian Intervention (TranS-C). This is a Hybrid Type 1 randomized controlled trial. The 8 CMHC clinic sites have been allocated by cluster randomization to Standard TranS-C (8x50min sessions) or Adapted TranS-C (4x20min sessions). Adapted TranS-C is hypothesized to better “fit” the CMHC context relative to Standard TranS-C. Then, within each CMHC site, patients are randomized to immediate TranS-C or to Usual Care followed by Delayed Treatment with TranS-C (UC-DT). Patients are assessed pre, mid and post-treatment and 6-months later (6FU). Providers are assessed after attending a training and before and after delivering TranS-C. The study is comprised of 3 phases: an Implementation Phase, a Train-the-Trainer (TTT) Phase and a Sustainment Phase. If the current 4-year timeline is retained, we anticipate that we can address the original specific aims for the study, which focused on the Implementation Phase outcomes. However, the TTT phase has been impacted more heavily by the pandemic, due to the increased demands on community providers, which has slowed progress. This administrative supplement is submitted to capitalize on this rare opportunity to use the momentum that is only just building to generate high quality data on TTT by extending the study for 1- year. This is important because (a) there is a dearth of literature on TTT and (b) research on TTT has potential to help sustain EBTs long-term. It is anticipated that the “end-game”, with the additional year, would be a sample of 60 providers who have been trained within the TTT network and 130 patients who have been treated by providers who have been trained within the TTT network. This sample size will enable more fully powered analyses on both patient- and provider-levels, offering some of the most comprehensive research on TTT to date. Of note, furthering scientific knowledge on TTT requires considerable “infrastructure” in the form of the Implementation Phase. Hence, the proposed administrative supplement will preserve the contribution to knowledge on TTT, an opportunity that would require considerable additional future investment to recreate.