Study: Does working night shifts increase breast cancer risk?

IN DEPTH REVIEW OF RESEARCH

Study background:

The World Health Organization’s International Agency for Research on Cancer (IARC) released a statement in 2007 saying that “Shift work that involves circadian disruption is a probable (breast) carcinogen.” In other words, night shift work that requires women to work when they would normally sleep increases breast cancer risk.  However, the statement was primarily based on studies that looked at animals after their normal sleep-wake pattern (circadian rhythm) was disrupted, with minimal evidence from human studies.

Ruth Travis and colleagues from the University of Oxford and other institutions published work in the Journal of the National Cancer Institute in October 2016 regarding whether night shift work increases breast cancer risk in women.  

Researchers of this study wanted to know:

Does working night shifts increase a woman’s breast cancer risk?  

Population(s) looked at in the study:

The study authors collected data from the following three cohorts:

• The Million Women Study surveyed 522,246 women.
  • Participants were asked whether they had ever regularly worked at night or on night shifts. The women who answered “yes” were asked about how long they worked night shifts and the nature of their work.
• The UK Biobank cohort surveyed 251,045 women.
  • Participants were asked about their employment; women who were employed were asked if they worked night shifts either “never/rarely,” “sometimes,” “usually,” or “always.”
• The EPIC-Oxford cohort surveyed 22,559 women.
  • Participants were asked whether they had ever regularly worked at night or on night shifts. The women who answered “yes” were asked about how long they worked night shifts and the nature of their work.

Women who had invasive cancer of any type or in situ breast cancer (DCIS or LCIS) before they began working night shifts were not included in this study.

Study findings: 

1. Women who worked night shifts had the same rate of breast cancer as women who did not.
   • Even women who worked night shifts for 20 or more years did not have an increased risk of developing breast cancer compared to women who had never worked night shifts.
2. Additionally, the authors also pooled the data from 10 past studies done by different researchers.  Here they also saw no difference in breast cancer rates between women who worked night shifts and those who did not.

Limitations:

Although the study included many women, relatively few (1,000) reported working night shifts long term. The study authors recognize that small increases in breast cancer risk for women who worked night shifts long term cannot be ruled out because of this small sample size. Additionally, other differences were observed between women who worked night shifts and those who did not: women who worked night shifts were slightly more likely to be obese, smoke, and to take medications to help them sleep. Finally, the meta-analysis portion of the research study where the researchers pooled information from past studies only included retrospective studies, meaning that the researchers of the studies used for this analysis did not collect their own data, and so they could not control for all factors.                    

Conclusions:

This study suggests that night shift work does not increase a woman’s risk of breast cancer. While more work should be done to confirm these findings, women who are concerned about their breast cancer risk should talk to their health care providers.

Posted 3/24/17

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