BACKGROUND: Epidemiological studies provide evidence for an association between shift work and sleep problems but often lack precise exposure and outcome data. OBJECTIVE: To investigate the risk of first-time physician-diagnosed sleep disorder in nursing staff using register-based data of shift work and health outcomes. DESIGN: A prospective cohort study with a 4.5-year follow-up. PARTICIPANTS: 25,639 healthcare employees (nurses including midwives, nursing assistants, and related care professions) employed for at least six months between 2012 and 2016 by Region Stockholm were included. METHODS: Information on hour-by-hour and day-by-day working hours was obtained from a computerised employee register. Physician-diagnosed sleep disorders (N = 326) were obtained from an outpatient register from January 2013 to June 2017. Discrete time proportional hazards models were used to estimate hazard ratios (HR) adjusting for age, sex, country of birth and profession. RESULTS: An increased risk of physician-diagnosed sleep disorder was seen among those who, during the preceding six months, only or frequently (>
66 times) worked night shifts compared to those who did not work nights (HR 1.70, 95 % CI 1.17-2.43 and HR 1.77, 95 % CI 1.21-2.50 respectively). The associations were supported by a dose-response pattern (p = 0.0006). Among those who work night shifts, the risk associated with 9-12 times of three or more consecutive nights was of borderline statistical significance (HR 1.66, 95 % CI 0.98-2.81). Frequently (>
39 times the last six months) having quick returns from night shifts (<
28 h) showed an almost 2-fold increased risk of physician-diagnosed sleep disorder compared to those who had few (<
8 times in the last six months) quick returns from night shifts (HR 1.89, 95 % CI 1.05-3.52). Quick returns from afternoon shifts in non-night workers (<
11 h) did not significantly increase the risk of physician-diagnosed sleep disorder. CONCLUSION: The results indicate that intensive night shift work, especially frequent and consecutive night shifts, is associated with an increased risk of physician-diagnosed sleep disorder in nursing staff. This study raises awareness of the need to acknowledge objective in addition to subjective health outcomes in relation to shift work.