In 2007, the International Agency for Research on Cancer|World Health Organization (IARC|WHO) concluded that the effects of shift work on the disruption of normal circadian rhythm had a probable link to breast cancer. IARC suggest that our endogenous 24-hour body clocks may be subject to interference by factors such as exposure to light at night, and it’s impact on melatonin levels may be linked to breast cancer. However, a recent meta-analysis led by Dr Ruth C. Travis published in the Journal of the National Cancer Institute concludes that night shift work may actually have very little effect on breast cancer risk.

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The UK Health and Safety Executive called for further epidemiological research on breast cancer after the review by IARC in 2007. In response to this, Travis et al undertook a meta-analysis of ten prospective studies worldwide (1.4 million women) to examine this area further. Travis et al contributed to three of these studies by introducing meticulous survey questions to three different groups of women (0.8 million combined) who were then followed-up according to National Health Service (NHS) records providing information on cancer registration and deaths. The studies included women who never worked night shifts, women who had worked night shifts for 10 years, 20 years and over 30 years. Some provided information on the standard length of their night shifts (e.g. 10 hours) to deduce whether or not longer shifts would have a higher association with breast cancer incidence.

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The meta-analysis concluded that there is virtually no link between night shift work and breast cancer incidence. This means that there is little to no difference in risk between those who have done night shift work and those who have not. However, although this is the largest study in this area to date, more research is required in terms of longer follow-up periods. What Travis et al also found was that women who worked night shifts were more likely to have characteristics such as obesity and tobacco use; factors which could ultimately contribute to a higher risk of breast cancer. These results are a reminder of the importance of keeping a tobacco-free environment, remaining physically active, making careful dietary choices and being cautious of alcohol consumption.