Depression prevention in digital cognitive behavioral therapy for insomnia: Is rumination a mediator?

Background There has been growing support for digital Cognitive Behavioral Therapy (dCBT-I) as a scalable intervention that both reduces insomnia and prevents depression. However, the mechanisms by which dCBT-I reduces and prevents depression is less clear. Methods This was a randomized controlled trial with two parallel arms: dCBT-I (N=358), or online sleep education as the control condition (N=300). Outcome variables were measured at pre-treatment, post-treatment, and one-year follow-up, and included the Insomnia Severity Index (ISI), the Quick Inventory of Depressive Symptomatology (QIDS-SR₁₆), and the Perseverative Thinking Questionnaire (PTQ). The analyses tested change in PTQ scores as a mediator for post-treatment insomnia, post-treatment depression, and incident depression at one-year follow-up. Results Reductions in rumination (PTQ) were significantly larger in the dCBT-I condition compared to control. Results also showed that reductions in rumination significantly mediated the improvement in post-treatment insomnia severity (proportional effect = 11%) and post-treatment depression severity (proportional effect = 19%) associated with the dCBT-I condition. Finally, reductions in rumination also significantly mediated the prevention of clinically significant depression via dCBT-I (proportional effect = 42%). Limitations Depression was measured with a validated self-report instrument instead of clinical interviews. Durability of results beyond one-year follow-up should also be tested in future research. Conclusions Results provide evidence that rumination is an important mechanism in how dCBT-I reduces and prevents depression.