'No link' between night shifts and breast cancer risk

October 08, 2016 481

"Working night shifts has ‘little or no effect’ on a woman’s risk of developing breast cancer, new research suggests," BBC News reports. This was the finding of a new study looking at data from 10 different countries.

The review pooled the evidence of three large UK-based studies, each of which found no significant link between night shift work for any number of years and risk of breast cancer. This study acted on a previous 2007 review by the World Health Organization which identified seven studies suggesting sleep disruption may be carcinogenic (cancer causing) to humans.

However, when the results of those seven studies were pooled with the three UK studies there was still no significant link.

These are all observational studies, so the possibility that other health and lifestyle factors associated with night-shift work – such as obesity or smoking – could increase breast cancer risk still can’t be ruled out.

One hypothesis, which we looked at in 2013, is that disruption to the "sleep hormone" melatonin could influence the risk of breast cancer by its effect on oestrogen production; another hormone associated with breast cancer.

If you work night shifts, you can offset your risk of breast cancer and other cancers by quitting smoking if you smoke, maintaining a healthy weight, eating a healthy, balanced diet, moderating your consumption of alcohol and taking regular exercise.

Where did the story come from?

The study was carried out by researchers from the University of Oxford and was funded by the UK Health and Safety Executive, Cancer Research UK and the Medical Research Council. The study was published in the peer-reviewed Journal of the National Cancer Institute.

Some of the authors are affiliated with GlaxoSmithKline, a large pharmaceutical company with breast cancer drugs in the market.

Both BBC News and the Daily Mail’s coverage on this study is accurate.

What kind of research was this?

This systematic review aimed to identify evidence from UK prospective cohort studies that could inform the hypothesis that long-term night shift work is associated with risk of breast cancer.

A systematic review is the best way of compiling the evidence from all relevant studies examining the link between an exposure and an outcome. With the nature of this question all studies have to be observational, rather than randomised controlled trials, for practical and ethical reasons.

What did the research involve?

The researchers identified three prospective UK studies of post-menopausal women: The Million Women Study (522,246 participants), EPIC-Oxford (22,559), and the UK Biobank (251,045).

In all three studies, participants were asked about their employment and whether their job involved working night shifts. The answers were categorised into:

never/rarely
sometimes
usually
always

The participants were followed via records linked to the NHS Central Registers which provide information on cancer registrations and deaths. The outcomes of interest in this analysis were the first diagnosis of breast cancer or death from breast cancer.

The data was then analysed to compare breast cancer incidence based on the different frequencies of night shift work. Further analysis examined associations between disease incidence and duration of night shift work:

fewer than 10 years
10-19 years
20 or more years

The data was adjusted for a number of potential confounders such as family history for breast cancer and use of oral contraceptives and hormone replacement therapy.

What were the basic results?

Overall, across studies there was no significant difference in risk of breast cancer between the women who worked night shifts and those who hadn’t.

In the Million Women Study, 4,809 breast cancers were diagnosed in the years following the questionnaire. There was no statistically significant difference in breast cancer risk between those who had never worked at night (4,136 cancer cases) and those who had (673 cancers) (relative risk [RR] 1.00, 95% confidence interval [CI] 0.92 to 1.08).
In this study there was also no difference in breast cancer risk between those who had never worked a night shift and those who had worked them for more than 20 years (RR 1.00, 95% CI 0.81 to 1.23).
In EPIC-Oxford, 181 breast cancers were diagnosed. Night shift work had no significant effect on risk of breast cancer (RR 1.07, 95% CI 0.71 to 1.62).
In the UK Biobank, 2,720 breast cancers were diagnosed. Again there was no significant difference in risk depending on night shift work (RR 0.78, 95% CI 0.61 to 1.00).

When combining the results of these studies with the seven non-UK studies included in the previous 2007 World Health Organization review, there was still no evidence that night shift work was associated with breast cancer (RR 0.99, 95% CI 0.95 to 1.03).

How did the researchers interpret the results?

The researchers concluded: "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."

Conclusion

This review aimed to answer the question of whether long-term night shift work could be associated with risk of breast cancer. It found no evidence for a significant link between the two.

This review had a good design and strength in its large population size. However, as these are observational studies, it’s worth bearing in mind there may still be confounding from other health and lifestyle factors. For example, the authors highlight that the women who had worked night shifts were more likely to be obese, to be smokers, take medication to help them sleep, and prefer evenings to mornings.

However, as some of the results were of borderline statistical significance, the researchers state that a possible link cannot be ruled out. They also consider that a link could be found with longer follow-ups and larger study populations.

Nevertheless, the evidence we have – both from these three UK studies and international studies – suggests night shift work won’t increase your breast cancer risk. Although night shift work has the potential to disrupt the sleep cycle, if working them regularly, individuals may adjust to a routine which allows their body clock to adjust and for them to still have enough sleep.