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

Background: 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. Method: 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. Results: 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. Conclusions: 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.