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Summary
Permanent night workers in Danish hospitals did not have a higher injury risk than permanent day workers. Employees whose schedules combined night work with evening or rotating shifts had higher injury rates.
The study examined 192,711 hospital employees between 2007 and 2018, using payroll records containing exact working hours and injury data from the Danish National Patient Register. The employees contributed 298.5 million observation days, during which 87,185 injuries were recorded.
Compared with permanent night workers, employees working evening/night schedules had a 37% higher adjusted injury rate. The same increase was found among those working day/evening/night schedules. Permanent day workers had almost the same adjusted injury rate as permanent night workers.
The findings suggest that mixed schedules involving night work may deserve more attention than permanent night work alone. However, the study was observational and included injuries occurring both at work and outside work, so it does not show that particular shift patterns caused the differences.
Aim and Context
Night work can affect sleep, alertness and circadian rhythms, but it is less clear whether permanent night work carries the same injury risk as rotating or mixed schedules.
The researchers examined whether permanent night workers had a different injury rate from employees working permanent days, evenings or combinations of day, evening and night shifts.
They expected permanent night work to be associated with higher injury risk. The findings did not support that expectation.
Methodology
The researchers conducted a register-based cohort study using the Danish Working Hour Database.
For each observation day, an employee's work pattern during the preceding 365 days was used to classify them into permanent day, permanent evening, permanent night or mixed shift groups.
Injuries were identified from the Danish National Patient Register. The outcome included injuries requiring secondary healthcare whether they occurred at work or elsewhere.
Poisson regression with generalised estimating equations was used to compare injury rates. The main analysis adjusted for sex, age and job type.
A further model included recent working-hour characteristics such as shift length, quick returns and the number of day, evening and night shifts worked during the previous seven days.
Separate analyses were also carried out for nurses and assistant nurses.
Permanent Night and Day Workers Had Similar Injury Rates
Permanent day workers had an adjusted incidence rate ratio of 0.99 compared with permanent night workers, with a 95% confidence interval of 0.93 to 1.07.
This means the two groups had broadly similar adjusted injury rates.
The result does not mean that permanent night work has the same health effects as permanent day work. The study examined injuries and did not assess the wider consequences of long-term night working.
Mixed Shift Schedules Had Higher Injury Rates
The highest adjusted rates were found among employees whose schedules combined different shift types.
Evening/night workers had an incidence rate ratio of 1.37 compared with permanent night workers. Employees working day/evening/night schedules also had an IRR of 1.37.
Day/night workers had an IRR of 1.19, while day/evening workers had an IRR of 1.22. Permanent evening workers had an IRR of 1.17.
The overall pattern therefore showed higher rates across several mixed schedules, rather than a simple difference between people working nights and people working days.
Recent Working Hours Did Not Fully Explain the Difference
The researchers tested whether working patterns during the previous seven days could account for the results.
After adjustment for recent factors including shift length and quick returns, the differences became smaller but did not disappear.
The IRR for evening/night workers fell from 1.37 to 1.27. For day/evening/night workers it fell from 1.37 to 1.25.
This indicates that recent working hours did not explain the whole association, although the study could not identify the underlying reason.
Nurses and Assistant Nurses Showed Different Patterns
Among nurses, most work schedules had similar injury rates. The main exception was day/evening/night work, which was associated with a 20% higher adjusted injury rate than permanent night work.
The pattern was clearer among assistant nurses.
Every other schedule group had a higher adjusted injury rate than permanent night-working assistant nurses. The IRRs ranged from 1.13 for day/night work to 1.51 for evening/night work.
These differences suggest that occupation and work tasks may influence how shift patterns relate to injury risk.
Why Might Permanent Night Workers Have Lower Rates?
The study could not establish why permanent night workers had lower injury rates than several mixed-shift groups.
The authors discuss possible explanations including adaptation to regular night work, differences in work tasks and worker selection.
People who tolerate permanent nights poorly may move to another schedule, while those who cope better may remain. Permanent night workers could therefore differ from other shift workers in ways that were not fully measured.
These explanations remain uncertain because the study did not directly test them.
Limits of Evidence
The research was observational, so differences between groups do not prove that the shift schedules caused the injuries.
Working conditions and job responsibilities may also differ between schedules. Adjustment for job type cannot account for every difference in the work being performed.
The injury data included incidents outside work as well as occupational injuries. The findings should therefore be described as injury risk among hospital employees rather than workplace accident rates alone.
The study involved Danish hospital workers, so the results cannot automatically be applied to sectors with different working conditions and hazards.
Takeaways for Practice
- Employers should avoid treating all forms of night work as having the same injury profile.
- In this study, permanent night workers had similar adjusted injury rates to permanent day workers, while several mixed schedules involving nights had higher rates.
- Hospitals and other organisations operating around the clock may therefore need to examine rotating and mixed schedules when reviewing fatigue, recovery time and injury data.
- The findings do not justify treating permanent night work as risk-free or replacing rotating shifts with permanent nights without considering other health and operational effects.
Read the Full Research Study:
Nabe-Nielsen, K., Aagaard, A., Larsen, A. D., Nielsen, H. B., Hansen, J., Hansen, Å. M., Kolstad, H. A., Vestergaard, J. M., & Garde, A. H. Permanent night work and risk of injuries: A register-based cohort study using payroll data. Scandinavian Journal of Work, Environment & Health. https://www.ncbi.nlm.nih.gov/pubmed/41983679