The 10-year study of more than 100,000 women by the Institute for Cancer Research in London found no clear evidence that working nights increased a woman’s chance of developing the disease.
The findings, published today in the British Journal of Cancer, come as global evidence on night shift work and cancer – including breast cancer risk – is set to be reviewed by the International Agency for Research on Cancer (IRAC) this summer.
For decades, it has been suggested that night shift work may increase a woman’s breast cancer risk and in 2007 IRAC concluded that disruption to the body clock caused by working nights was “probably carcinogenic”.
However, recent studies, including a 2016 meta-analysis of research evidence, have suggested there may be no impact on breast cancer risk after all.
The new study, funded by the charity Breast Cancer Now, is based on data from 102,869 women who were followed up over 9.5 years, on average, to identify who went on to develop breast cancer.
Researchers gathered a wide range of information about the job histories and working patterns of the women, who had all signed up to take part in the Breast Cancer Now Generations Study.
Of the study group, 17.5% – nearly 18,000 women – reported being a night shift worker in the past 10 years.
The researchers also gathered information on a range of known breast cancer risk factors, including obesity, levels of physical activity, alcohol consumption and family history.
“We hope these findings will help reassure the hundreds of thousands of women working night shifts”
In all, 2,059 out of the 102,869 women went on to develop breast cancer, but the study found no overall link between night shift work and the likelihood of this happening.
The study authors also found no significant difference in risk in relation to the type of night shift work, the age at which women started working nights or whether night shift work was started before or after their first pregnancy.
There was a statistically significant link with average hours worked per week, but no association between any other variables.
The researchers said the finding on hours per week alone was not supported by previous evidence or any proposed biological explanation.
It has been suggested that a possible link between shift work and increased breast cancer risk could be explained by exposure to light at night disrupting the body’s internal clock.
This could cause suppression of the sleep hormone melatonin and raise oestrogen levels in the body.
However, another recent analysis by the same authors concluded that exposure to light at night while sleeping does not increase a woman’s risk of developing breast cancer.
“In our new study, we found no overall link between women having done night shift work in the last 10 years and their risk of breast cancer – regardless of the different types of work they did involving night shifts, and the age at which they started such work,” said study co-author Michael Jones.
“Although night shifts may have other effects on people’s health, and we still don’t know the effect of a person’s body clock being disturbed for very long periods of time, it is reassuring to see more evidence suggesting that night shifts are not linked with a higher risk of breast cancer,” he added.
Baroness Delyth Morgan, chief executive of Breast Cancer Now, hoped the findings would indeed reassure the many female employees who currently work nights.
Baroness Delyth Morgan
She said: “We hope these findings will help reassure the hundreds of thousands of women working night shifts that it’s unlikely their job patterns are increasing their risk of breast cancer.
“This question has been widely debated in recent decades and has understandably caused concern, and it’s encouraging that the evidence now suggests night shift work has no impact on breast cancer risk,” she said.
She stressed that breast cancer risk was affected by a combination of factors, including genetics and lifestyle, but there was “never one single cause of the disease”.
“With many contributing factors, it’s vital we support more women to do what they can to help shift the odds in their favour,” she added.
This included maintaining a healthy weight, keeping physically active and drinking less alcohol.