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Another Study Suggests Night Work Raises Risk

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Several studies have suggested a link between night shift work and higher breast cancer risk. Now a French study has found that women who worked the night shift were 35% more likely to be diagnosed with breast cancer compared to women who never worked night shifts.

The study was published online June 12, 2012 in the International Journal of Cancer. Read the abstract of “Night work and breast cancer: a population-based case-control study in France (the CECILE study).”

In the CECILE study, researchers compared the work histories of 1,232 French women who had been diagnosed with breast cancer to the work histories of 1,317 similar women who hadn’t been diagnosed with breast cancer.

Overall, women who had any history of working at night were 35% more likely than women who never worked at night to have been diagnosed with breast cancer. The increase in risk was linked to the number of years the women worked at night and when in a woman’s life she worked at night. Compared to women who never worked at night:

  • women who worked at night for 4.5 or more years were 40% more likely to have been diagnosed with breast cancer
  • women who worked at night for 4 or more years before their first pregnancy were almost twice as likely to have been diagnosed with breast cancer

It’s not clear why working at night influenced risk the most in women who worked night shifts before they first became pregnant. Breast cells become more mature during and after a woman’s first pregnancy. It’s possible that because the breast cells were immature before a first pregnancy, they were more vulnerable to factors that can encourage breast cancer to develop.

Doctors aren’t sure what’s causing the apparent 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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