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949 Shiftwork and breast cancer: epidemiology, burden, and implications for prevention
  1. Manisha Pahwa1,
  2. France Labrèche2,3,
  3. M Anne Harris1,4,5,
  4. Joanne Kim1,6,
  5. Chaojie Song1,
  6. Paul A Demers1,5,7
  1. 1Occupational Cancer Research Centre, Cancer Care Ontario, Toronto, Canada
  2. 2Institut de recherche Robert-Sauvé en santé et en Sécurité du travail, Montréal, Canada
  3. 3School of Public Health, Université de Montréal, Montréal, Canada
  4. 4School of Occupational and Public Health, Ryerson University, Toronto, Canada
  5. 5Dalla Lana School of Public Health, University of Toronto, Toronto, Canada
  6. 6Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Canada
  7. 7CAREX Canada, Simon Fraser University, Burnaby, Canada


Introduction Approximately one in five workers globally work night, evening, or rotating shifts. Shiftwork involving circadian disruption is a probable carcinogen for breast cancer. Our objective was to synthesise the current state of the epidemiological literature, report on shiftwork-associated breast cancer burden in Canada, and discuss implications for prevention.

Methods A search was conducted for meta-analyses accompanied by a systematic review, published from 2010–2017, that included at least one meta-risk estimate (mRE) for breast cancer associated with any permanent/rotating night work exposure metric. For each included meta-analysis, heterogeneity values were extracted and an eight-point checklist was used to evaluate quality. An attributable fraction (AF) range for breast cancer, based on mREs from high quality meta-analyses and Canadian shiftwork survey data, was calculated using Levin’s equation.

Results Seven meta-analyses collectively included 30 cohort and case-control studies spanning 1996–2016. Most reported statistically significant heterogeneity. In 5 meta-analyses that scored ≥6 points on the quality assessment checklist, mREs for ever/never night shiftwork exposure ranged from 1.15 (95% confidence interval [CI]: 1.05 to 1.25, n=9 studies) to 1.40 (95% CI: 1.13 to 1.73, n=9 studies). Using these mREs as lower and upper values in Levin’s equation, the AF for breast cancer among the 1.5 million Canadian women who ever worked night/rotating shifts during 1961–2001 ranged from 2.04%–5.23%. This corresponds to an estimated 460–1180 annual incident breast cancers probably due to shiftwork; nearly half (200–510) are diagnosed among women in health care and social assistance.

Discussion Summaries of 20 years of epidemiological evidence support shiftwork as a probable breast carcinogen, but considerable heterogeneity between studies poses a challenge for precisely evaluating breast cancer risk and burden. Given the potentially substantial burden of breast cancer due to shiftwork, applied research on workplace-based prevention of circadian disruption is acutely needed to identify effective solutions for sectors where shiftwork prevalence is high.

  • Burden of disease
  • breast cancer prevention
  • women
  • work
  • and health

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