CBT-I is recommended as the primary treatment for people with both depression and insomnia. Adding psychological depression treatment alongside CBT-I may not be helpful and could be burdensome.

This 12-week study with a six-month follow-up aimed to ascertain whether combining Cognitive Behavioral Therapy for Insomnia (CBT-I) with Cognitive Behavioral Therapy for Depression (CBT-D), without extending treatment duration, results in greater reductions in both insomnia and depression compared to CBT-D with a placebo insomnia intervention.

Employing therapist-guided internet-delivered treatments, this study was carried out in a psychiatric unit. A total of 126 volunteers were detected with major depression and insomnia disorder by physicians. The enrolled subjects were segregated into two groups:

• Combined treatment group (n=64): Treated with CBT-I + CBT-D
• Control group (n=62): Treated with CBT-D + Placebo insomnia intervention

The key outcome measures were self-rating scales: the Insomnia Severity Index (ISI) and the Montgomery-Åsberg Depression Rating Scale (MADRS-S).

When compared to the control group, the combined treatment group demonstrated specific effects on insomnia severity (p = 0.007) but did not exhibit greater efficacy in minimizing the severity of depression. In this double-blind, randomized controlled trial, the within-group effect sizes (Cohen's d) at post-treatment and at six months were: MADRS-S 0.97 and 1.12 (combined), 0.88 and 0.89 (control); ISI 1.40 and 1.42 (combined treatment), 0.95 and 1.00 (control).

It was recommended that CBT-I must be prioritized for patients with comorbid insomnia and depression, possibly as the primary treatment choice. The addition of a psychological depression treatment alongside CBT-I might be overly burdensome and may not confer additional benefits.

CBT-I yielded significant specific effects on insomnia severity and outperformed the control intervention in this aspect. However, both treatments showed similar effects on depression severity, indicating that combining CBT-I with CBT-D did not lead to improved depression outcomes when compared to the control intervention.