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Special populations: women who work night shifts or who have done so in the past
The World Health Organization’s International Agency for Research on Cancer (IARC) classified night shift work as a possible risk factor for breast cancer in 2007, although the majority of the evidence for this claim came from studies of animals after their normal sleep-wake cycle was disrupted. The authors of this study surveyed women from three different cohorts to examine whether night shift work can increase a woman’s breast cancer risk. (3/24/17)
Whether working night shifts increases a woman’s risk of breast cancer.
In 2007, the World Health Organization’s International Agency for Research on Cancer (IARC) classified night shift work as a breast cancer risk factor. However, most of the evidence used to make this statement was based on research studies of animals after their normal sleep-wake cycle (circadian rhythm) was disrupted. As many women work night shifts for their occupation, it is important to understand whether this is a risk factor for breast cancer development in humans.
For some women, night shift work is unavoidable and even preferred. This study suggests that it does not significantly increase a woman’s risk of developing breast cancer—women who worked night shifts were no more likely to develop breast cancer than women who didn’t. Because this study looked at all women, it is not known how night shift work affects women who are already at high risk for breast cancer. All women, regardless of whether or not they do night shift work, should strive to live a healthy lifestyle that includes regular exercise, limited alcohol intake, and a nutritionally balanced diet, as these actions generally lower cancer risk.
Medical News Today
Daily Mail UK
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.
Does working night shifts increase a woman’s breast cancer risk?
The study authors collected data from the following three cohorts:
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.
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.
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.
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Staif, K, Baan, K, Grosse, Y, et al. “Carcinogenicity of shift-work, painting, and fire-fighting,” Lancet Oncology, 8(12), p.1065-1066, December 2007.
Travis RC, Balkwill A, Fensom GK, et al. “Night shift work and breast cancer incidence: three prospective studies and meta-analysis of published studies.” Journal of the National Cancer Institute, 2016, 108(12).
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