Working at night is associated with a greater risk of breast cancer in women

September 14, 2018 by Martin Lasalle, University of Montreal
Credit: CC0 Public Domain

Women who work at night, especially during pre-menopause, may be at greater risk of developing breast cancer.

This is the finding of a new analysis of surveys of 13,000 women from five countries conducted by a team of researchers that includes Anne Grundy, a research associate at Université de Montréal's Department of Social and Preventive Medicine, part of the School of Public Health.

The surveys in Australia, Canada, France, Germany and Spain looked at nearly 6,100 women who had been diagnosed with and approximately 7,000 who had no diagnosis. Participants answered self-administered questionnaires or telephone interviews about their occupation and about risk factors for cancer.

The average age of the women varied between 55 and 59, depending on the country, and the proportion working nights also varied widely, from 6 per cent in Germany to 17 per cent in Australia. In Canada, the proportion was 16.7 per cent.

12- to 80-per-cent higher risk

Published in the April issue of the European Journal of Epidemiology, the study found that the risk of developing ER-positive breast cancer (that is, of tumours associated with estrogen receptors) and/or HER2 breast cancer increases with the number of hours worked per , as well as the number of years spent on the night shift.

However, the risk seems to diminish two years after going off the night shift.

"Women who work at least three hours between midnight and 5 a.m. run a 12 per cent greater risk of developing breast cancer than women who have never worked at night," said Grundy. "Among pre-menopausal women, the risk associated with working at night increases to 26 per cent."

Night workers who work shifts longer than 10 hours have a 36-per-cent increased risk of breast cancer, again compared to women who have never worked nights.

The risk is as high as 80 per cent among women who work night shifts in excess of 10 hours for more than three nights per week.

"Women who were still working nights at the time of the study had a breast cancer risk that was 26-per-cent higher than those who had stopped working at night at least two years previously," said Grundy.

Risk associated with a melatonin deficiency?

The analysis of the five surveys did not specifically consider respondents' job type (night or otherwise), although the Canadian survey, done in 2013, did compare women working in healthcare and other sectors.

"The risk associated with breast cancer and night work varied little among respondents, regardless of job type," said Grundy. "Although we are not fully certain, we believe that this risk could be related to the hypothesis that night work disrupts circadian rhythm and inhibits the secretion of , which may protect against cancer."

Grundy now hopes to explore the effect of shift work on women's risk of breast . "We need to go further in our research so that labour policies ultimately take into account this for , and so that companies take preventive action and adjust work schedules."

Explore further: Night shift work and breast cancer risk

More information: Emilie Cordina-Duverger et al. Night shift work and breast cancer: a pooled analysis of population-based case–control studies with complete work history, European Journal of Epidemiology (2018). DOI: 10.1007/s10654-018-0368-x

Related Stories

Night shift work and breast cancer risk

October 6, 2016
Despite an assessment in 2007 indicating that night shift work was probably carcinogenic, data from three new studies and from a review of currently available evidence, published in the Journal of the National Cancer Institute, ...

Female night shift workers may have increased risk of common cancers

January 8, 2018
Night shift work was associated with women having an increased risk of breast, skin, and gastrointestinal cancer, according to a meta-analysis.

Long term night shifts linked to doubling of breast cancer risk

July 1, 2013
Shift work has been suggested as a risk factor for breast cancer, but there has been some doubt about the strength of the findings, largely because of issues around the assessment of exposure and the failure to capture the ...

Does night work put women's health at risk?

June 19, 2012
Breast cancer is the number one cause of female mortality. It affects 100 out of 100,000 women per year in developed countries. Each year, more than 1.3 million new cases are diagnosed, 53,000 of these in France.

Outdoor light at night linked with increased breast cancer risk in women

August 17, 2017
Women who live in areas with higher levels of outdoor light at night may be at higher risk for breast cancer than those living in areas with lower levels, according to a large long-term study from Harvard T.H. Chan School ...

Night shifts may be linked to increased ovarian cancer risk

March 14, 2013
Working night shifts might increase the risk of developing ovarian cancer, indicates research published in Occupational and Environmental Medicine.

Recommended for you

New mechanism controlling the master cancer regulator uncovered

November 21, 2018
Who regulates the key regulator? The Research Center for Molecular Medicine of the Austrian Academy of Sciences reports online in the journal Science about a newly discovered mechanism by which RAS proteins, central to cancer ...

Researchers stop spread of cancer in mice by blocking specific molecules

November 21, 2018
Melanoma skin cancer tumors grow larger and are more likely to metastasize due to interactions between a pair of molecules, according to experiments in mice and human cells. The results may restore the potential for a type ...

'Druggable' cancer target found in pathway regulating organ size

November 20, 2018
It's known that cancer involves unchecked cell growth and that a biological pathway that regulates organ size, known at the Hippo pathway, is also involved in cancer. It's further known that a major player in this pathway, ...

A study suggests that epigenetic treatments could trigger the development of aggressive tumours

November 20, 2018
A study headed by the Institute for Research in Biomedicine (IRB Barcelona) and published in the journal Nature Cell Biology examined whether the opening of chromatin (a complex formed by DNA bound to proteins) is the factor ...

Redefining colorectal cancer subtypes

November 20, 2018
There is a long-standing belief that colorectal cancer (CRC), which causes some 50,000 deaths in the United States each year, can be categorized into distinct molecular subtypes. In a paper published recently in the journal Genome ...

Proposed cancer treatment may boost lung cancer stem cells, study warns

November 20, 2018
Epigenetic therapies—targeting enzymes that alter what genes are turned on or off in a cell—are of growing interest in the cancer field as a way of making a cancer less aggressive or less malignant. Researchers at Boston ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.