Night shift work has up until recently been considered a risk factor for breast cancer, but a recent study led by researchers at the University of Oxford (1) has re-opened this debate and has cast doubt on whether night shift workers really are at increased risk of the disease.

In this science blog we consider the background to the current research findings and whether we can now be certain night-shift work is not a breast cancer risk factor.

The research by Travis and colleagues was commissioned by the UK Health and Safety Executive, as part of an investigation to try to establish whether working at night increases the risk of chronic diseases such as breast cancer. It analysed data from a range of international and UK research papers which included information on over 1.4 million post-menopausal women, many of whom were nurses. Each study independently found night-shift work does not increase breast cancer risk. Analysis of all 10 studies concluded that night shift work, even of 20 years or more duration, has “little or no effect on breast cancer incidence”.

It is clear that the review was well designed, included a large population size, and took into consideration many known risk factors, but as with all observational studies there may be other health and lifestyle factors that were not considered in the analysis. The authors themselves state that some of the results were of borderline statistical significance. They also mention that the women in the UK studies who worked night shifts were more likely to be obese, smoke, and take medications to help them sleep, which are all potentially confounding factors. It is also possible that only certain sectors of the population (e.g. pre-menopausal women) are at increased risk of breast cancer from night-shift work, so a possible link cannot be ruled out entirely.

What is the evidence that night-shift work does increase breast cancer risk?

A 2007 IARC report classified night-shift work as a “probable cause” of breast cancer, based on evidence from animal research and limited evidence of an effect on human breast cancer (2). Many subsequent reports have supported this conclusion (3). For example research that involved over 2 million Chinese women (4) found an increased risk of both breast cancer incidence and mortality, which was greater with longer duration of night shift work. Another study, which considered hormonal and HER2 status of breast cancers, found that night-shift work increased breast cancer risk, but only of hormonal and HER2 positive cancers in pre-menopausal women (5) However, other research (e.g. 6, 7 and this study) have found no association between breast cancer risk and night-shift work.

Why might night shift work increase the risk of breast cancer?

The reason night-shift work might increase breast cancer risk is thought to be associated with production of the hormone melatonin (8). This hormone is kept in check by light and produced and released into the bloodstream in response to darkness. Melatonin helps control circadian rhythms (our sort of 24 hour biological clock that helps us to sleep and wake). Night shift workers going from a day environment to an artificial light environment at night have lower levels of melatonin. Melatonin has known anti-cancer properties (9). These include inhibition of oestrogen-induced cell proliferation, inhibition of tumour cell growth and increased tumour cell death. Melatonin is thought to affect the initiation, promotion and progression of breast cancer.

This new study does suggests that night-shift work may not increase breast cancer risk. But it is difficult to ignore previous studies that reach a different conclusion or the biological role that melatonin plays. Therefore we cannot be certain that there is no association between night shift work and breast cancer under any circumstances and believe further research is needed before we can rule out any link conclusively.

It is important that night shift workers look at other ways to help reduce their risk of breast cancer – for example by ensuring they eat a healthy diet (including plenty of fruit), reduce their alcohol consumption, get plenty of exercise and reduce their exposure to harmful chemicals. If you are concerned about getting enough sleep and need some helpful tips and advice on how to adjust your body clock, there are plenty of websites that contain further information see, for example, www.sleepcouncil.org.uk .

What is clear is that breast cancer incidence continues to rise in the population as a whole, which suggests that there is some general environmental influence that affects us all. We welcome all research which investigates the role of potential breast cancer risk factors and urge government bodies such as the HSE to invest further in research that identifies the role of universal potential risk factors such as environmental chemicals.

 

References:

1. Travis, R. C. et al. (2016). Night Shift Work and Breast Cancer Incidence: Three Prospective Studies and Meta-analysis of Published Studies. JNCI Journal of the National Cancer Institute 108(12): djw169. https://www.ncbi.nlm.nih.gov/pubmed/27758828

2. WHO and IARC (2010). IARC Working Group on the Evaluation of Carcinogenic Risks to Humans: Volume 98 Painting, firefighting and shiftwork. https://monographs.iarc.fr/ENG/Monographs/vol98/mono98.pdf (accessed October 18, 2016)

3. Fenga, C. (2016). Occupational exposure and risk of breast cancer (Review). Biomedical Reports 4: 282-292. https://www.ncbi.nlm.nih.gov/pubmed/26998264

4. Lin, X. et al. (2015). Night shift work increases morbidity of breast cancer and all-cause mortality: a meta-analysis of 16 prospective cohort studies. Sleep Medicine 16(11): 1381–1387. https://www.ncbi.nlm.nih.gov/pubmed/26498240

5. Cordina-Duverger, E. et al. (2016). Night work and breast cancer risk defined by human epidermal growth factor receptor-2 (HER2) and hormone receptor status: A population-based case-control study in France. Chronobiology International 33(6): 783-787. https://www.ncbi.nlm.nih.gov/pubmed/27078711

6. Li, W. et al. (2015). Shift work and breast cancer among women textile workers in Shanghai, China. Cancer Causes Control 26: 143‑150. https://www.ncbi.nlm.nih.gov/pubmed/25421377

7. Ijaz, S. et al. (2013). Night-shift work and breast cancer – a systematic review and meta-analysis. Scandinavian Journal of Work, Environmental and Health 39(5):431-447. https://www.ncbi.nlm.nih.gov/pubmed/23804277

8. Hill, S. M. et al. (2015). Melatonin: an Inhibitor of Breast Cancer. Endocrine Related Cancer 22(3): R183–R204. https://www.ncbi.nlm.nih.gov/pubmed/25876649

9. Hill, S. M. et al. (2015). ibid


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