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Women Who Work Night Shifts Have Higher Breast Cancer Risk

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Studies have suggested a link between night shift work and increased breast cancer risk. A 2012 Danish study found that women who worked the night shift were 40% more likely to be diagnosed with breast cancer compared to women who never worked the night shift.

The study was published online May 29, 2012 in Occupational and Environmental Medicine. Read Nested case-control study of night shift work and breast cancer risk among women in the Danish military.

The researchers surveyed 18,551 women born between 1929 and 1958 and reviewed their work histories. All the women worked in some way for the Danish military between 1964 and 1999. All the women were 75 or younger when they completed the questionnaire. From this information, the researchers calculated how much day shift vs. night shift work the women did.

Using a cancer database -- the Danish Cancer Registry -- the researchers determined which women had been diagnosed with breast cancer. They then looked for links between the women’s shift work history and their risk of being diagnosed with breast cancer.

Overall, women who had worked night shifts were 40% more likely to be diagnosed with breast cancer compared to women who never worked night shifts.

Breast cancer risk increased the most in women who frequently (or always) worked night shifts for a long time. For these women, breast cancer risk was more than doubled compared to women who never worked night shifts.

On the other hand, women who worked one or two night shifts per week for fewer than 6 years had the same breast cancer risk as women who never worked night shifts.

Doctors aren’t sure what’s causing the link between regular night shift work and breast cancer risk. Some doctors think that lower levels of melatonin, a hormone made in the brain, could explain the higher risk. Melatonin plays a role in regulating the body’s sleep cycle and may also help regulate cell growth and repair. People who don’t sleep at night (when it’s dark outside) tend to have lower melatonin levels. Lower melatonin levels may lead to patterns of breast cell growth and repair that make breast cancer more likely to develop.

Some doctors think the higher breast cancer risk seen in night shift workers may be related to lifestyle factors. For example, some research has found that obesity, smoking, and drinking alcohol -- all of which have been linked to higher breast cancer risk -- are more common in women who work night shifts.

In our 24-hour world, many people have no choice but to work night shifts; not working night shifts to minimize breast cancer risk isn’t a realistic option for many women. For these women, the possible link between breast cancer risk and night shift work makes it more important to make other lifestyle changes -- eating a healthy diet, exercising, not smoking, avoiding alcohol -- that can decrease breast cancer risk.

In the Breastcancer.org Lower Your Risk section, you can learn much more about breast cancer risk and the steps you can take to keep your risk of breast cancer as low as it can be.

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