For a long time it has been unclear whether there is a link between working night shifts and breast cancer.

Back in 2007, the International Agency for Research on Cancer (IARC) concluded that night shift work ‘probably’ caused cancer, based on the available evidence at the time.

This classification doesn’t mean that people who work night shifts will probably develop breast cancer. Instead, it refers to how good the evidence is that a direct link exists between the two.

In 2009 and 2012 breast cancer hit the headlines again.

And now new research, published last week by our scientists, has put this issue under the spotlight once more.

The team, based at the University of Oxford, found that working night shifts doesn’t increase the risk of breast cancer after all.

So why has science seemingly changed its mind?

Shedding light on the issue

Night shift work might seem like an odd thing to link to breast cancer. But scientists previously thought that the effect of light at night on our internal ‘body clock’ might increase the risk of cancer.

And when scientists tested this theory out in the lab by looking at the effects of light at night on animals, their results suggested there could be a link.

And it’s because of this evidence from the animal studies, coupled with the limited evidence from human studies, that IARC made the decision to classify shift work as a ‘probable’ cause of breast cancer.

This was reported widely in the media at the time, and in 2009 the Danish government went so far as to offer compensation to women who had worked night shifts and later developed breast cancer.

But the small number of studies that found a link between shift work and breast cancer in people had their drawbacks.

Although shift work did seem to increase breast cancer risk, some studies didn’t account for other risk factors, like body weight and drinking alcohol. Others didn’t use enough people to draw solid conclusions. And the different studies often used different definitions for ‘night shift’, meaning it’s difficult to compare the results.

On top of this, many studies asked women to think back over their lives and remember their working histories and other factors, which may have affected the findings.

So although there was quite a bit of research linking night shifts and breast cancer, it wasn’t possible to say for sure that shift work itself is a definite cause of cancer.

What was needed was research that improved upon previous studies, for example by involving more people from a wide range of job types. And taking known causes of cancer, like body weight and drinking alcohol, into account.

And it’s the results of that research, funded by the UK’s Health and Safety Executive, along with Cancer Research UK and the Medical Research Council, that were published last week.

So what did they find?

Shifting evidence

The study, led by Dr Ruth Travis, a Cancer Research UK-funded scientist at the University of Oxford, combined the results of 10 different studies from around the world.

We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer

– Dr Ruth Travis

Three of these were brand new, and included key information about working hours among a group of UK women who were then followed up via NHS records.

When all 10 studies were combined, the scientists were able to draw conclusions based on data from over 1.4 million women. And they found that the link just wasn’t there.

“We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer,” says Travis. And this held true in the three new UK studies and “when we combined results from all 10 studies that had published relevant data,” she adds.

The scientists even looked at whether nurses in particular were at a higher risk of breast cancer, in an attempt to explain the findings of some of the older studies in humans that specifically looked at nurses who worked night shifts. There was no link here either -nurses were no more likely to be diagnosed with breast cancer than women who did other jobs.

Even though this study has many strengths, it wasn’t perfect – no study is. The UK women weren’t followed up for very long after they were asked about their working lives, meaning that there wasn’t much time for new cases of breast cancer to develop.

But based on all the research to date we don’t think there is enough good evidence to suggest that night shift work causes breast cancer.

So will IARC be altering its stance on shift work?

Véronique Terrasse, a press officer from IARC, said: “The classifications are based on all available published studies, not just one.” But she added that “shift work is a high priority for re-evaluation”.

You can read more about IARC’s plans for reviewing the evidence around shift work in a report by their Advisory Group.

Could night shift work be unhealthy for other reasons?

Even though night shift work itself doesn’t appear to be a cause of breast cancer, women who work night shifts may be more likely to be diagnosed with breast cancer because of other differences.

The new research also found that women working night shifts are more likely to be overweight or obese than women who don’t work night shifts. And this can increase the risk of breast cancer, as well as other serious diseases.

This may be because their working pattern makes it more difficult for shift workers to do things like shop for and cook healthy food, or take part in regular physical activity. So more work is needed to understand the best way to make sure healthy choices are easily available for everyone in our 24 hour society, including those who work to provide it.

What does this mean for women who work night shifts?

Crucially, when it comes to a direct link between shift work and breast cancer, this study is the largest of its kind, which suggests the results are solid.

So we hope this reassures women who work night shifts.

“Although I do enjoy night shifts, I definitely feel it can affect your mental and physical health,” says Lindsay Mcowat, a junior doctor who works night shifts. “This new research leaves me feeling relieved that I don’t have to worry about breast cancer too.”

And regardless of work schedules, keeping a healthy weight, reducing alcohol intake and being physically active can all help to stack the odds of avoiding cancer in your favour.

Rachel Orritt is a health information officer at Cancer Research UK