Chronotype and insomnia symptoms in the incidence and persistence of depressive symptoms: An 8-year longitudinal cohort study.

BACKGROUND: Previous research findings on the association between chronotype and depressive symptoms have relied on cross-sectional studies, and the temporal relationship remains unclear. This study aimed to examine whether insomnia symptoms modify the longitudinal association between chronotype and depressive symptoms. METHODS: This study was an 8-year longitudinal study of 1549 adults who completed the baseline survey. Insomnia symptoms and chronotype were assessed using the Insomnia Severity Index and the Circadian Energy Scale. The presence of depressive symptoms was defined as a Patient Health Questionnaire-9 score of 10 or higher. RESULTS: Evening chronotype and insomnia symptoms were associated with depressive symptoms (evening: odds ratio [OR] = 1.73, 95% confidence interval [CI] = 1.08 to 2.78; insomnia symptoms: OR = 4.78, 95% CI = 3.89 to 5.87). In the incidence of depressive symptoms, the interaction between chronotype and insomnia symptoms was significant, with the presence of insomnia symptoms increasing the risk across all chronotypes (morning: relative risk [RR] = 5.20, 95% CI = 1.97 to 13.75, risk difference [RD] = 15.73%; intermediate: RR = 3.39, 95% CI = 2.54 to 4.52, RD = 12.04%; evening: RR = 1.70, 95% CI = 1.11 to 2.59, RD = 7.28%). In the persistence of depressive symptoms, only insomnia symptoms were significant (RR = 1.74, 95% CI [1.34, 2.27], RD = 18.67%). CONCLUSIONS: These findings suggest that the association between chronotype and depressive symptoms differs by insomnia status and is specific to the incidence of depressive symptoms, but not the persistence of depressive symptoms.