Why Sleep is Key: Poor Sleep Quality is a Mechanism for the Bidirectional Relationship between Major Depressive Disorder and Generalized Anxiety Disorder Across 18 Years

Generalized anxiety disorder (GAD) and major depressive disorder (MDD) reliably precede and predict one another. However, there is insufficient data on mediators through which the longitudinal GAD-MDD association unfold. Based on insomnia theories, such as the hyperarousal model of sleep, we tested the degree to which poor global sleep quality functioned as a mediator of the prospective bidirectional anxiety-depression relationship.Participants were 3,294 community-dwelling adults who partook in three measurement waves nine years apart. The Composite International Diagnostic Interview-Short Form assessed GAD and MDD in-person at baseline (Time 1 [T1]), Time 2 (T2; nine years after T1), and 18 years later (T3). T2 global sleep quality was measured using the multiple-domain Pittsburgh Sleep Quality Index self-report at T2. We used longitudinal structural equation modeling mediation analyses.Analyses showed that higher T1 MDD and GAD severity individually predicted lower T2 global sleep quality (Cohen's d = -0.561 to -0.480) and less T2 global sleep quality, thereby forecasted both higher T3 MDD and GAD (d = -0.275 to -0.190). Poorer T2 global sleep quality significantly mediated the T1 GAD-T3 MDD relation, explaining 41% of the association. Worse global sleep quality at T2 also significantly mediated the T1 MDD-T3 GAD association, mediating 11% of the T1 MDD-T3 GAD pathway. The results remained similar after controlling for multiple sociodemographic and clinical variables.Findings offer evidence for transdiagnostic theories of sleep and insomnia. Theoretical and clinical implications, such as prioritizing sleep improvement in cognitive-behavioral therapies, are also discussed.