Night Shift Work and Breast Cancer Incidence

Hopeful news on this topic, with a large-scale survey. A lot of people worry about trying to get complete darkness at night--looks like this may not matter.

Abstract below. You can read the full article at the link with (free) Medscape membership.

Night Shift Work and Breast Cancer Incidence

Three Prospective Studies and Meta-analysis of Published Studies

Ruth C. Travis; Angela Balkwill; Georgina K. Fensom; Paul N. Appleby; Gillian K. Reeves; Xiao-Si Wang; Andrew W. Roddam; Toral Gathani; Richard Peto; Jane Green; Timothy J. Key; Valerie Beral

Disclosures

J Natl Cancer Inst. 2016;108(12)

Abstract and Introduction

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.

Introduction

Light at night can suppress melatonin secretion, and it has been proposed that night shift work could increase breast cancer incidence.^[1] In a review of the evidence available in 2007, the World Health Organization's International Agency for Research on Cancer (IARC) classified shift work that involves circadian disruption as a probable (breast) carcinogen, based on sufficient evidence from animal studies but only limited evidence of an effect on human breast cancer.^[2]

The available human evidence reviewed by IARC in 2007 was characterised as inadequate for assessing moderate risks reliably partly because of potential confounding and inconsistent definitions of shift work and partly because most studies were retrospective in design, comparing responses from women already diagnosed with breast cancer with those from unaffected women. Some retrospective results might have been moderately biased by differential recall, and/or by differential participation in the studies between women who had and had not worked night shifts.^[3–5] The only prospective information available in 2007 that directly compared breast cancer incidence in women reporting night shift work with that in other women was from two studies of US nurses, and elevated breast cancer rates were reported for 20 or more years of night shift work.^[6,7]

Since the IARC review, results from five further prospective studies have been published (2 in urban China and 3 in continental Europe)^[8–12] and several meta-analyses have been done,^[13–18] although the most recent included results from only five of the prospective studies.^[18] To provide reliable epidemiological evidence, with minimum methodological bias, on any relationship between night shift work and breast cancer incidence, we now report first results from an additional three prospective studies and an updated meta-analysis of findings from all prospective studies, now 10 in total.