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 were 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 pretreatment, weeks 1-5, and week 10. 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 10-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 (95% CI: -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 provides 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.Clinical Trial CompRest-a Comparison Between Sleep Compression and Sleep Restriction for Treating Insomnia. URL: https://clinicaltrials.gov/study/NCT02743338?term=NCT02743338&rank=1Registration NCT02743338.