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More Research Links Working at Night to Higher Breast Cancer Risk

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Several studies have suggested a link between working at night and higher breast cancer risk. Now a Canadian study also suggests that women who worked the night shift for 30 years or more were twice as likely to be diagnosed with breast cancer as women who never worked the night shift.

The research was published online on July 1, 2013 by Occupational and Environmental Medicine. Read the abstract of “Increased risk of breast cancer associated with long-term shift work in Canada.”

The study looked at 1,134 women diagnosed with breast cancer from British Columbia and 1,179 women who were of similar ages and hadn’t been diagnosed with any type of cancer except non-melanoma skin cancer. Non-melanoma skin cancer is basal or squamous cell skin cancer and isn’t considered as serious as melanoma skin cancer. Non-melanoma skin cancers rarely spread to other parts of the body. The women who hadn’t been diagnosed were either from British Columbia or from Kingston, Ontario.

About 33% of the women in both groups reported that they had worked at a job where at least half their time was spent working at evening or night. This included both rotating schedules and permanent night shift schedules.

The researchers found no link between working at night and breast cancer risk for women who worked at night for 3 months to 29 years. But breast cancer risk was doubled in women who had worked at night for 30 or more years. The results were similar for women who worked in healthcare, such as doctors and nurses, and women who worked in other, non-health fields.

While the results of this study are troubling, there are some weaknesses in the research:

  • The women had to remember and report their work history for the past 30 years. While many women may be able to do this accurately, it could be that some of the women misreported how much time they spent working at night.
  • The women in the study all came from two areas in Canada. It could be that other factors specific to British Columbia and Kingston, Ontario were affecting the women’s breast cancer risk.

Doctors aren’t sure what’s causing the apparent link between 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 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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