In its 2007 review, the World Health Organization’s International Agency for Research on Cancer categorized shift work that involves disruption of the circadian rhythm as a probable carcinogen. Although this conclusion was based on animal studies, the messaging to the public was that working nights caused or contributed to the development of breast cancer.
To examine whether or not night shift work could increase the risk of developing breast cancer, researchers at Oxford University in the United Kingdom (UK) followed women enrolled in 3 large UK studies and estimated the relative risks of breast cancer among women who reported that they worked night shifts versus those who reported that they did not work at night.
The 522,246 women answered questions on shift work and were followed over time. No increase in breast cancer risk associated with night shift work—including long-term night shifts—was found in any of these studies.
In a subsequent meta-analysis, the researchers combined results from the 3 UK studies with those from all 7 previously published prospective studies (2 in the US, 2 in China, 2 in Sweden, and 1 in the Netherlands). The 10 studies included 1.4 million women among whom 4660 breast cancers occurred in women who reported ever having done night shift work. The meta-analysis confirmed that women who worked night shifts were not more likely to develop breast cancer.
The study findings are available here
Travis RC, Balkwill A, Fensom GA, et al. Night shift work and breast cancer incidence: three prospective studies and meta-analysis of published studies [published online October 6, 2016]. J Natl Cancer Inst.