Permanent night work and risk of injuries: A register-based cohort study using payroll data

OBJECTIVE: Shift work is associated with a higher injury risk, but the optimal way of organizing night work remains debated. This study examined whether the injury risk among permanent night workers differs from that of employees working other types of work schedules with or without night work.

METHODS: This register-based cohort study used payroll data from the Danish Working Hour Database over a 12-year period (2007-2018), with daily information on working hours among all hospital employees in Denmark. Work schedules were categorized according to the proportion of night, evening, and day shifts worked in the preceding 365 days. Hospital-treated injuries were identified using the Danish National Patient Register. Poisson regression with generalized estimating equations was used to estimate incidence rate ratios (IRR) for injuries across work schedules. Main analyses were adjusted for sex, age, and job type.

RESULTS: Among 192 711 employees contributing 298.5 million observation days, we identified 87 185 injuries. Permanent night workers had a lower injury risk compared with all other groups of shift workers and a similar risk as permanent day workers. Relative to permanent night workers, the observed injury risk was higher among evening/night workers [IRR 1.37, 95% confidence interval (CI) 1.23-1.53] and day/evening/night workers (IRR 1.37, 95% CI 1.28-1.47).

CONCLUSION: Permanent night workers had lower risk of injuries than permanent evening workers and workers in 2- or 3-shift schedules. Differences in tasks, adaptation, and selection may contribute to this pattern. Injury prevention efforts should prioritize workers exposed to night shifts in combination with other shift types.