Background Chronotype and insomnia have been related to the development and to an unfavourable course of depression. However, the mutual relationship of both risk factors is as yet unclear, especially in acute, clinically manifest depressive disorders. Aims The present study was carried out to elucidate the separate direct and indirect influence of chronotype and poor sleep quality on depression severity in patients hospitalized for depression. Methods Depression severity (BDI-II), chronotype (Morningness-Eveningness Questionnaire), and subjective sleep quality (Pittsburgh Sleep Quality Index total score) were assessed concurrently in inpatients with a depressive syndrome and insomnia during routine treatment. Correlations, multiple regression and bootstrapping methods for testing mediation models were applied to assess the independent direct and indirect effects of chronotype and sleep quality on depression severity, after adjusting for effects of age and gender. Results Data from 57 consecutively admitted patients (88% with major depression) were analyzed (68% women, mean age 41 ± 13 years). Significant correlations between morningness-eveningness (p <0.05) or sleep quality (p <0.01) and depression severity were found; in a multiple regression model comprising chronotype, sleep quality, age and gender, only chronotype (p <0.05) and sleep disturbances (p <0.01) remained as independent significant concurrent predictors of depression severity (R(2) = 0.184, p <0.01). Two mediation models revealed no significant results. Conclusions Eveningness and poor subjective sleep quality were independently and directly associated with higher depression severity in inpatients with depressive syndromes. Chronotype and sleep quality should be taken into account not only in risk assessment and prevention but also in hospitalized patients to develop and improve treatment options.
