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Night shift work, long working hours and dementia: a longitudinal study of the Danish Work Environment Cohort Study
  1. Kirsten Nabe-Nielsen1,
  2. Åse Marie Hansen1,
  3. Kazi Ishtiak-Ahmed1,
  4. Matias Brødsgaard Grynderup1,
  5. Finn Gyntelberg2,
  6. Sabrina Islamoska1,
  7. Erik Lykke Mortensen1,3,
  8. Thien Kieu Thi Phung4,
  9. Naja Hulvej Rod1,
  10. Gunhild Waldemar4,
  11. Rudi G J Westendorp1,3,
  12. Anne Helene Garde2
  1. 1 Department of Public Health, University of Copenhagen, Copenhagen, Denmark
  2. 2 National Research Centre for the Working Environment, Copenhagen, Denmark
  3. 3 Center for Healthy Aging, University of Copenhagen, Copenhagen, Denmark
  4. 4 Danish Dementia Research Centre, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark
  1. Correspondence to Kirsten Nabe-Nielsen; nabe{at}


Objective Shift work and long working hours are potential risk factors for dementia, but previous studies on shift work, long working hours and dementia are sparse and their findings are conflicting. Therefore, we investigated the effect of night shift work and long working hours on dementia.

Design A longitudinal study.

Setting Denmark.

Participants 3435 occupationally active men and women from the general working population.

Methods Work schedule covered day work (reference) and shift schedules without/with night work. Working hours covered <27, 28–36, 37 (reference), 38–44, and ≥45 hours/week. As the primary outcome, we used register-based information about dementia, and estimated incidence rate ratios (IRR) and 95% CI. Estimates were adjusted for gender, age, psychosocial work factors and cardiovascular risk factors.

Results We identified 85 dementia cases during a mean of 9.8 years of follow-up. We found a positive, but statistically insignificant association between night shift work and dementia (IRR=2.01; 95% CI: 0.87-4.65). Post hoc analyses indicated that this was only due to a higher risk in permanent night workers (IRR=3.25; 95% CI: 1.35-7.83). The dementia risk was also significantly higher among participants working 38–44 hours/week (IRR=2.08; 95% CI: 1.11-3.90) compared with those working 37 hours/week. We found no indications of a higher risk of dementia in participants working <37 hours/week or ≥45 hours/week.

Conclusion We did not find arguments that night shift work or long working hours increased dementia risk in general. However, we found a higher risk of dementia in specific subgroups, that is, permanent night workers and employees with moderately longer weekly working hours than the standard.

  • dementia
  • epidemiology
  • public health

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  • Contributors KN-N, AHG, FG, ELM, TKTP, NHR, GW and ÅMH conceived and designed the study. KN-N and KI-A conducted the data analyses. KN-N, ÅMH, KI-A, MBG, FG, SI, ELM, TKTP, NHR, GW, RGJW and AHG interpreted the results. KN-N drafted the article. KN-N, ÅMH, KI-A, MBG, FG, SI, ELM, TKTP, NHR, GW, RGJW and AHG critically revised the article. KN-N, ÅMH, KI-A, MBG, FG, SI, ELM, TKTP, NHR, GW, RGJW and AHG gave the final approval for the version to be published.

  • Funding This study was supported by a grant from The Danish Working Environment Research Fund (10- 2015-03 20150017498). The fund has not had any influence on the design of the study, statistical analyses, interpretation of the findings or the conclusion of this study.

  • Competing interests None declared.

  • Ethics approval The study has been notified to the Danish data protection authorities and adheres to the regulations that ensure the protection of personal data. According to the Danish legislation, this study did not need approval from the local Committee for Health Research Ethics.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data sharing statement Deidentified participant data from the DWECS survey may under certain conditions be available from The National Research Centre for the Working Environment,Copenhagen, Denmark (contact:

  • Patient consent for publication Not required.

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