Night Work and Breast Cancer: Updated

Photo Courtesy of Sascha Kohlmann

Here’s good news for the 15 million Americans who work night shifts and other irregular or rotating schedules. A new review that compiled data from over 1.4 million women challenges the 2007 World Health Organization (WHO) assertion that breast cancer risks increase for those working nights.

New Data on the Breast Cancer & Night Shift Link

The study “Night Shift Work and Breast Cancer Incidence: Three Prospective Studies and Meta-analysis of Published Studies” was published in October of 2016 in the Journal of the National Cancer Institute by Dr. Ruth Travis, of the Cancer Epidemiology Unit at Oxford University in the United Kingdom and her colleagues. It analyzes data from 3 large-scale studies out of the U.K—the Million Women Study (522,246 women), the UK Biobank (251,045 women), and the European Prospective Investigation into Cancer and Nutrition-Oxford (EPIC-Oxford) study (22,559 women)—along with 7 smaller studies specifically assessing the link between night shift work and breast cancer.

Participants in the 10 studies all provided information about their work schedules and were followed up with for diagnoses of breast cancer. Among the participants who worked at night, 4,660 cases of breast cancer were recorded. Compared to women who had never worked night shifts, the relative risks were calculated to be:

  • .99 for women who had worked night shifts at some point
  • 1.00 (or equal) for women who had worked night shifts for 20-30 years
  • 1.01 for women who had worked night shifts for 30 years or longer

Researchers concluded night shift workers, regardless of the length of employment, have the same risk as those who do not work the night shift.

Previous Studies: Melatonin, Estrogen & Cancer Risks

The 2016 review was conducted as a direct response to the WHO’s conclusion, issued in 2007, “mainly from animal evidence, that shift work involving circadian disruption is probably carcinogenic to humans.” The International Agency for Research on Cancer (IARC), part of the WHO, included night shift work in their list of carcinogens.

In 2012, a study in the journal Occupational and Environmental Medicine suggested a causal link between night shift work and increased risk for breast cancer. This previous study reported as high as a 40% increased risk for night shift workers, pointing to the disruption in sleep as the cause for this increased risk. The authors of the study pointed out that that light at night suppressed the body’s natural release of melatonin, which in turn could increase the release of estrogen, a known risk factor for breast cancer.

Night Shift Work Effects on Obesity & Smoking

While, according to the new review, night shift work does not directly cause breast cancer, a connection between night shift work and other breast cancer risk factors is not ruled out.

Smoking and obesity, have both been shown to increase breast cancer risk, and were both more prevalent among night shift workers. 19.1% of night-shift worker participants were smokers compared to 13.3% of those who had never worked night shifts. And 19.2% of night shift workers were obese compared to 14.7% of non-night-shift workers.

What should you do if you work the night shift? Good news—don’t worry! You are not at higher risk for breast cancer. However, don’t let your evening hours get in the way of your breast health. Make sure to maintain a healthy diet, get regular exercise, maintain a healthy weight, quit smoking, and moderate your consumption of alcohol. Working nights can make it a challenge to schedule appointments, but don’t forget your yearly mammograms and clinical breast exams.