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 (May 1, 2024–May 31, 2025) recruited shift-working nurses at a tertiary hospital in Guangxi, China. Circadian rhythm types were assessed using the Circadian Type Inventory (CTI; flexibility–rigidity, FR; languidness–vigorousness, LV), sleep quality by the Pittsburgh Sleep Quality Index (PSQI), and depressive symptoms by the Patient Health Questionnaire-9 (PHQ-9). Objective shift work demand data over 4 weeks (night shifts, total shift hours, workload exposure) were extracted from the nursing management system. Generalized linear models (GLMs), nonlinear curve fitting, and Monte Carlo simulation were used. Results: Among 288 nurses, 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) predicted poorer sleep quality. Poorer sleep quality (β = 0.314), greater flexibility (β = −0.129), greater languidness (β = 0.159), and interactions between sleep quality and flexibility (β = 0.091) and between languidness and shift work hours (β = 0.069) predicted depressive symptoms. Nonlinear analysis suggested a threshold: >24 shift work hours/4 weeks linked to poorer sleep quality. Simulations demonstrated distinct dose–response patterns in sleep and depressive symptoms by circadian type and shift work demands. Conclusion: Circadian rhythm types and shift work demands jointly shape sleep quality and depressive symptoms, with distinct dose–response patterns, supporting circadian-informed shift scheduling to improve nurse sleep and mental health.