Abstract
Rates of major depressive disorder (MDD) are increasing globally, in part due to the coronavirus disease 2019 pandemic, contributing to disease burden. It has long been known that insomnia is intricately connected with depression as indicated by greater depression severity and lower treatment response. Furthermore, insomnia is a significant risk factor for new-onset depression. Treatment of insomnia is thus a logical target for prevention of incidents and recurrent MDD. This systematic review sought to evaluate the current evidence for the preventive effects of insomnia treatment on depression onset. A database search yielded 186 studies, six of which met criteria for inclusion in this review. All of the studies utilized cognitive behavioral treatment for insomnia (CBT-I) as the target intervention and most delivered treatment via a digital platform. Four of the studies found significantly lower rates of MDD onset in those who received CBT-I compared to a control condition. The two remaining studies failed to confirm these effects in primary analyses but secondary analyses suggested evidence of a preventive effect. There was significant methodologic heterogeneity across studies in terms of sample selection, outcomes, and follow-up periods, limiting the ability to draw firm conclusions. The evidence overall is in the direction of insomnia treatment reducing the risk for onset of MDD, but further research is warranted.