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    Does night shift work increase risk of breast cancer?

    Night shift work for women appears to have little or no effect on breast cancer incidence, according to results of a new study published in the Journal of the National Cancer Institute. The findings are at odds with a World Health Organization review that found night shift work was particularly likely to increase the risk of breast cancer.

    Researchers looked at 542,246 women in the Million Women Study, 22,559 women in EPIC-Oxford, and 251,045 women in UK Biobank who had answered questions on shift work and were followed for incident cancer. Heterogeneity was assessed using Cox regression, which yielded multivariable-adjusted breast cancer incidence rate ratios (RRs) and 95% confidence intervals (CIs).

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    In the Million Women Study 673 women who reported night shift work developed breast cancer; 28 women who reported night shift work developed breast cancer in EPIC-Oxford; and 67 women who reported night shift work developed breast cancer in UK Biobank. The RRs for any versus no night shift work were 1.00 (95% CI = 0.92 to 1.08), 1.07 (95% CI = 0.71 to 1.62), and 0.78 (95% CI = 0.61 to 1.00). In the Million Women Study, the RR for 20 or more years of night shift work was 1.00 (95% CI = 0.81 to 1.23) and no statistically significant heterogeneity by breast cancer risk factors or sleep patterns was seen.

    The researchers concluded that night shift work appears to have either very little or no effect on the incidence of breast cancer.

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    Miranda Hester
    Ms. Hester is Content Specialist with Contemporary OB/GYN and Contemporary Pediatrics.


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