ObjectiveTo explore the mediating effect of sleep quality between somatic symptoms and severity of depression in patients with depression.MethodsA total of 384 drug-naive patients diagnosed with depression were recruited from the Department of Psychological Medicine of Zhongshan Hospital, Fudan University, during the period from February to August 2024. The severity of depression, somatic symptoms, and sleep quality were assessed using Patient Health Qusetionaire (PHQ)-9, PHQ-15, and Pittsburgh sleep quality index (PSQI), respectively. Based on the PHQ-15 scores, all participants were stratified into two groups: a mild somatic symptoms group（<10 points, n=136）and a moderate-to-severe somatic symptoms group（≥10 points, n=248）. Comparisons of sleep quality between the two groups were conducted, and partial correlation analysis was performed to examine the correlation between sleep quality and somatic symptoms. Additionally, linear regression and mediation analyses were conducted to investigate the mediating effect of sleep quality between somatic symptoms and severity of depression.ResultsThe PSQI scores in moderate-to-severe somatic symptoms group were significantly higher than those in mild somatic symptoms group (P<0.001). Partial correlation analysis indicated that, after controlling for depression severity, the positive correlation between PSQI and PHQ-15 scores remained significant in both groups (P<0.01). Regression analysis identified both sleep quality and somatic symptoms as predictors of severity of depression (P<0.001). Additionally, mediation analysis demonstrated that sleep quality partially mediated the relationship between somatic symptoms and severity of depression, accounting for 26.63% (0.090/0.338) of the total effect.ConclusionsIn patients with depression, sleep quality is associated with somatic symptoms, and both contribute to an increased risk of the severity of depression. Moreover, sleep quality plays a partial mediating effect between somatic symptoms and severity of depression, highlighting the importance of addressing sleep-related issues in the management of depression.