Shiftwork and Risk of Breast Cancer

Shiftwork, Breast Cancer Risk

Research has found a relationship between shiftwork, particularly shiftwork involving nights, and an increased risk for cancer. This has led the International Agency for Research on Cancer to conclude that shift work is a probable carcinogen.

For women, the specific concern is the risk of breast cancer.

Using data from the Nurses’ Health Study in the U.S., researchers found an increased risk of breast cancer. Several other studies in Finland and Norway came to the same conclusion.

These studies found that a number of factors were associated with the increased risk.

  • Exposure to light at night – as this suppresses melatonin, the hormone that regulates sleep and plays a role in tumor growth
  • Circadian disruption of sleep – may cause sleep deprivation which is a risk factor for cancer
  • Extent of exposure to night work – as this increases exposure to light, disruption of melatonin, sleep deprivation
  • Number of years doing shiftwork involving nights – with risk increasing significantly with 20 years of shiftwork

An epidemiological Canadian study published in 2012 considered shiftwork not only as an occupational factor associated with cancer, but also considered the outcomes when women were exposed to other known workplace carcinogens along with shiftwork.

Given all this data, several questions arise.

1.  Should women with exposure to shift work involving nights and a diagnosis of breast cancer be eligible for workplace compensation claims?

The answer is complex because one would have to consider other factors such as exposure to other non-workplace carcinogens, employee health behaviors, age and other breast cancer risk factors. Finland, however, has gone ahead and become the first country to establish such a compensation plan.

2. Is there a safe exposure level?

Again, the answer is complex. As the studies show, risk increases with exposure to night work. However, these studies assume a homogeneous group of shiftworkers. In fact, shiftworkers have different capacities for adapting to shiftwork and dealing with the challenges of disrupted sleep. Taking this into consideration when assigning shifts would decrease the extent of exposure.

3. Are employers liable for best practice shift schedules?

Since circadian disruption is one of the factors contributing to risk, risk could be reduced by ensuring that shift schedules maximize the opportunities for sleep and reduce sleep deprivation. In the same vein, employers could also minimize the need for overtime or call-in’s on days off as this would reduce sleep deprivation as well.

4. Should employees be implementing measures to reduce their risk?

Since exposure to light at night seems to be a factor that increases risk of cancer, should employees be implementing measures to reduce their exposure to blue light? An easy way to do this is to wear yellow glasses which block blue light and, therefore, do not suppress melatonin.


Institute for Work and Health, Issue Briefing April 2010 – summary of research

By Carolyn Schur | ,

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