STUDY OBJECTIVES: Insomnia disorder, affecting 10% of the population, poses a significant public health concern and is a risk-factor for many health issues. Cognitive behavioral therapy is first-choice treatment, but the key component - sleep restriction therapy - presents with side effects and adherence challenges. Sleep compression therapy, suggested as a potentially gentler alternative, has never been directly compared to sleep restriction therapy. METHODS: Single-blind trial at the Internet psychiatry clinic in Stockholm, Sweden. Patients with insomnia disorder randomized 1:1 to evaluate non-inferiority of sleep compression therapy to sleep restriction therapy in improving insomnia, and to compare important clinical aspects. Primary outcome: self-reported Insomnia Severity Index (ISI), assessed pre-treatment, week 1-5, and week ten. Non-inferiority analysis based on intent-to-treat analyses with multiple imputation and mixed effects models. RESULTS: Adults with insomnia (n=234
mean age 44.3 (SD 13.7) years, 173 (73.4%) female) received treatment as a ten-week highly structured, therapist-guided online program, to strengthen experimental integrity and treatment fidelity. Both treatments improved insomnia severity with large effects. Sleep compression therapy failed to show non-inferiority with a conservative limit of 1.6 ISI-points (CI-95% -0.01 - 1.70), gave statistically significantly smaller improvements (p = .006) and was associated with slower improvements despite better adherence and somewhat less side effects. CONCLUSIONS: This direct comparison and well-controlled trial provide empirically based support for clinicians to prioritize sleep restriction therapy over sleep compression therapy, while the latter can be a valid alternative when sleep restriction therapy cannot be used.