Health

Night Shift Doesn’t Cause Breast Cancer

Debunking a nearly-decade old study that has troubled many women

There are any number of factors that may increase a woman’s risk for breast cancer — but working the night shift isn’t among them.

Data from three new studies and from a review of currently available evidence indicate that night shift work has little or no effect on breast cancer incidence. The findings refute a 2007 assessment by the World Health Organization’s International Agency for Research on Cancer (IARC) indicating that night shift work was probably carcinogenic.

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The findings most likely also ease the minds of a lot of women who don’t work typical business hours and who have felt for years that they were putting their health at risk.

In a 2007 review, the World Health Organization’s International Agency for Research on Cancer categorized shift work that involves disruption of the circadian rhythm, people’s internal “body clock,” as a probable carcinogen. This was based on evidence about breast cancer in animal studies. At the time there was only limited evidence about breast cancer risk in humans.

The meta-analysis of three U.K. studies and seven previously published prospective studies included 1. 4 million women, among whom 4660 breast cancers occurred in those who reported ever having done night shift work.

Researchers in the new review found no increase in breast cancer risk associated with night shift work, including long-term night shifts, in any of these studies.

It’s good to see the revised data, said Dr. Alberto Bessudo, medical oncologist and researcher at California Cancer Associates (cCARE) in San Diego. He said there has never been a real concern about the third shift playing any real role in women’s risk for breast cancer. Having the theory disproved officially, however, is helpful for women.

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“The reality is, there are women who work at night who may have other factors that made it look like they had more breast cancer. For example, maybe a woman who works overnights is less likely to get pregnant — clearly we know women who get pregnant at a very late age or who never get pregnant have more breast cancer. Maybe people who work at night have other risk factors that have never been tested; maybe they take hormones in a different way. So I think it’s really good to see that this disproves fears that are really not true,” Bessudo told LifeZette.

What is worth keeping in mind is: Are there any genetic risk factors for breast cancer?

“Know your risk,” Bessudo said. “And get screened. Find cancer early so you can be cured.”

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He added that with any study, it’s important to be cautious of the information you’re given because often things get published without good scientific data. “This proves you need to wait for strong data to be published before you change your life because something is being commented on, yet it’s not complete,” said Bessudo.

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