Night Shift Work and Breast Cancer Incidence: Three Prospective Studies and Meta-analysis of Published Studies

Abstract

Background: It has been proposed that night shift work could increase breast cancer incidence. A 2007 World Health Organization review concluded, mainly from animal evidence, that shift work involving circadian disruption is probably carcinogenic to humans. We therefore aimed to generate prospective epidemiological evidence on night shift work and breast cancer incidence.Methods: Overall, 522 246 Million Women Study, 22 559 EPIC-Oxford, and 251 045 UK Biobank participants answered questions on shift work and were followed for incident cancer. Cox regression yielded multivariable-adjusted breast cancer incidence rate ratios (RRs) and 95% confidence intervals (CIs) for night shift work vs no night shift work, and likelihood ratio tests for interaction were used to assess heterogeneity. Our meta-analyses combined these and relative risks from the seven previously published prospective studies (1.4 million women in total), using inverse-variance weighted averages of the study-specific log RRs.Results: In the Million Women Study, EPIC-Oxford, and UK Biobank, respectively, 673, 28, and 67 women who reported night shift work developed breast cancer, and the RRs for any vs 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), with no statistically significant heterogeneity by sleep patterns or breast cancer risk factors. Our meta-analysis of all 10 prospective studies included 4660 breast cancers in women reporting night shift work; compared with other women, the combined relative risks were 0.99 (95% CI = 0.95 to 1.03) for any night shift work, 1.01 (95% CI = 0.93 to 1.10) for 20 or more years of night shift work, and 1.00 (95% CI = 0.87 to 1.14) for 30 or more years.Conclusions: The totality of the prospective evidence shows that night shift work, including long-term shift work, has little or no effect on breast cancer incidence.

Shift Work: Disrupted Circadian Rhythms and Sleep—Implications for Health and Well-being

“…Night shift work has also been found to increase the risk of other cancers, including lung, colon, bladder, prostate, rectal, and pancreatic cancer as well as non-Hodgkin’s lymphoma [61,62,63]. However, a number of studies have found no association between cancer and shift work [64,65,66,67]. The variability in these findings may be related to other contributing factors, such as individual differences in susceptibility to carcinogenesis, occupational exposure to carcinogenic agents, and lifestyle [68,69].…”

Sleep and circadian disruption and incident breast cancer risk: An evidence-based and theoretical review

“…However, as with previous meta-analyses, there was significant heterogeneity in the characterization of NSW as a risk factor across the included studies. Moreover, as was identified in a recently published response to the Travis et al meta-analysis, the baseline age of women in the paper’s 3 prospective studies ranged from early-50 s to upper-60 s with follow-up periods of only 2–3 years (Travis et al, 2016; Schernhammer, 2017). This makes it challenging to support the authors’ claim that the “melatonin hypothesis” for breast cancer risk lacks merit, but gives credence to the IARC Working Group’s {IARC, 2009} argument for establishing greater consistency in operationally defining NSW as a risk factor.…”

“…Kamdar and colleagues (Kamdar et al, 2013) did not find that participants working 8 or more years of NSW were at greater risk for developing breast cancer compared to non-shift workers, and Jia and colleagues (Jia et al, 2013) found that women working 15 or more years of NSW were not at increased risk above and beyond women who reported ever working night shift. The most recent meta-analysis (Travis et al, 2016) which included previously unpublished data from three prospective studies along with data from 7 published studies, found no significant associations among NSW and incident breast cancer risk (RR = 0.99, 95% CI = 0.95–1.03). However, as with previous meta-analyses, there was significant heterogeneity in the characterization of NSW as a risk factor across the included studies.…”

Shift work and cardiovascular disease – do the new studies add to our knowledge?

“…Since the association between shift work and chronic diseases like CVD and breast cancer attenuates after cessation of exposure (6, 7), restricting study populations to elderly cohorts of former shift workers -who have not been exposed for years -may overlook real associations. The recent Million Women Study (8), which found no association between shift work and cancer, included subjects of very advanced age. Starting follow-up at an older age may have resulted in the inclusion of more participants less susceptible to the effects of night shift work (7,9).…”