Night shifts linked to breast cancer in women? Probably – but not definitely

A Danish study into whether working night shifts could affect a woman’s risk of breast cancer is hitting the headlines today.

But (as is often the case), when you look beyond the headlines, the picture can become a little less clear.

So could working night shifts cause breast cancer?   The science says “probably”. But probably can be a bit of a tricky word: it isn’t no, but it isn’t really yes either.

So what’s going on? We’ve covered this issue before on this blog, so what does today’s study add to the picture?

The story so far

In 2007 the International Agency for Research on Cancer (IARC), which is part of the World Health Organisation, classified shift working that involves disrupting people’s daily ‘body clock’ cycles as a probable cause of cancer in humans.

That means they looked at all the available research in animals and humans, and summarised it all to decide that although there was enough evidence to show that light during night-time hours could cause cancer in animals, the evidence in humans was limited.

In other words, although many of the studies did show a link, the evidence wasn’t quite strong enough to say for sure that working night shifts could cause cancer in humans.

(For more thoughts on IARC’s classification system, read this post on mobile phones from 2011)

Shifting definitions

One reason for this uncertainty is that studies of the effect of night shifts on women’s health have used different definitions of what a night shift is. This has made it difficult to compare results of different studies.

On top of this, there are also differences in how these studies take account of other factors that are known to affect a woman’s risk of breast cancer, for example her weight, history of pregnancy and childbirth, or how much alcohol she drinks.

None of the studies is perfect, and they have all answered slightly different questions in slightly different ways. And this new study is no exception.

Not the final chapter

Today’s new study has some good points, such as looking at night shift work during all of a woman’s employment history.

And many of the previous studies looked at either nurses or flight attendants, so it had been difficult to say whether the observations were relevant to women in other types of job. This also left open the possibility that any link could be related to the actual job the women were doing, rather than the fact they were doing it at night.

But the new study has three key weaknesses.

Firstly, the number of women with breast cancer in the Danish study was small (only 132) – and only a third of them had actually done shift work before – which limits how reliably we can view the study results.

On top of this, the women were asked to think back over decades of their lives to gather information about their working patterns and other factors, which can make the results biased.

Thirdly, and most importantly, the statistical analysis in the report shows that the researchers can’t be totally confident that their findings aren’t just down to chance: most of the results they found weren’t ‘statistically significant’. This even includes the headline reported ‘40 per cent increase’ in risk.

Only when the researchers looked at the total number of hours of night shifts women worked over the course of their lives, but not the number of years they’d done these shifts for, did they find any significant results.

What if shifts do cause cancer?

Scientists are still trying to come to a consensus about whether shift work truly could increase breast cancer risk, and – if there is a link – to work out how big that effect might be, and how much shift work women could safely do. But based on the available scientific evidence, it just isn’t possible to give a reliable answer to those questions yet.

Some studies have tried to estimate how big shift work’s effect on breast cancer would be, if the link were real. A Cancer Research UK-funded study, published in 2011, estimated that around 2,200 cases a year in the UK could be due to shift work, if a link really does exist. To put that into perspective, the same study estimated around 3,000 cases of the disease were down to alcohol and 4,300 cases to being an unhealthy weight.

So where does that leave us?

The Health and Safety Executive has already commissioned a report into how shift work could affect the risk of various diseases, including cancer. It’s due to be published in 2015, and hopefully will find enough evidence to reach a definite conclusion.

But for now, the answer to the question of whether working night shifts could cause breast cancer is still “probably”. And although this study doesn’t provide the definitive answer, it does still add evidence to the pile – which will be needed if we’re going to resolve the question in future.

Sarah

Sarah Williams is a health information officer at Cancer Research UK

Reference

• Hanssen, J et al. Nested case-control study of night shift work and breast cancer risk among women in the Danish military. Occupational and Environmental Medicine (2012) DOI: 10.1136/oemed-2011-100240