Circadian rhythm types and shift work demands shape sleep quality and depressive symptoms in shift-working nurses

Objective To examine the predictive, moderating, and combined effects of circadian rhythm types and shift work demands on sleep quality and depressive symptoms among shift-working nurses. Methods A cross-sectional study was conducted between May 1, 2024, and May 31, 2025. Shift-working nurses were recruited using convenience sampling at a tertiary hospital in Guangxi, China. Circadian rhythm types, sleep quality and depressive symptoms were assessed using the Circadian Type Inventory CTI; including flexibility–rigidity (FR) and languidness–vigorousness (LV), the Pittsburgh Sleep Quality Index (PSQI) and the Patient Health Questionnaire-9 (PHQ-9). Objective data on shift work demands over a four-week period were extracted from the hospital nursing management system, including number of night shifts, total shift hours, and shift workload exposure. Generalized linear modeling (GLM), nonlinear curve fitting, and Monte Carlo simulation were used for data analysis. Results A total of 288 shift nurses were included. The GLMs showed that depressive symptoms ( β = 0.245), languidness ( β = 0.065), shift work hours ( β = 0.093), and body mass index ( β = −0.056) were significant predictors of poorer sleep quality. Poorer sleep quality ( β = 0.314), flexibility ( β = −0.129), languidness ( β = 0.159), and the interaction between sleep quality and flexibility ( β = 0.091), between languidness and shift work hours ( β = 0.069) significantly predicted depressive symptoms. Nonlinear analysis identified a potential threshold effect, with more than 24 shift work hours in 4 weeks linked to poorer sleep quality. Dynamic simulations demonstrated that the combined effects of circadian rhythm types and shift work demands corresponded to distinct dose–response patterns in sleep quality and depressive symptoms. Conclusion Circadian rhythm types and shift work demands jointly shape sleep quality and depressive symptoms in shift nurses, with distinct dose–response patterns. These findings highlight the importance of circadian-informed shift scheduling to improve sleep and mental health among shift nurses.