Background <p>Cognitive behavioral therapy for insomnia (CBT-I) has a high potential for improving insomnia and depressive symptoms; however, it is unclear whether the improvement of symptoms reaches the level of healthy people without these symptoms. We aimed to examine whether digital CBT-I can improve the insomnia and depression symptoms of workers to healthy levels.</p> Methods <p>The study included 752 workers who were divided into four groups by the use of the cutoff scores of insomnia and depression scales: insomnia alone, depression alone, combined insomnia with depression (COMB), and healthy. All groups were administered digital CBT-I for 2&#xa0;weeks, and changes were compared post-treatment and at 1- and 3-month follow-ups.</p> Results <p>A significant decrease in insomnia symptoms from post-treatment to the 3-month follow-up was found in the insomnia alone (Hedges’ g: 1.07–1.52) and COMB groups (g: 1.17–1.41). The COMB group also showed a significant decrease in depressive symptoms (g: 0.38–0.70). Moreover, there were significant differences in insomnia symptoms between both of the insomnia groups and the healthy group and in depressive symptoms between the COMB group and the healthy group, post-treatment and at the 1- and 3-month follow-ups.</p> Conclusions <p>Digital CBT-I effectively reduced insomnia and depressive symptoms but did not achieve the levels of healthy people within 3&#xa0;months.</p> <p>Trial registration.</p> <p>UMIN, UMIN000050353. Registered 15 February 2023—Retrospectively registered, umin.ac.jp/ctr.</p>