Research: shift work ‘probably or possibly carcinogenic’

“Shiftwork that involves circadian disruption is probably carcinogenic to humans.”
IARC, the cancer agency of the World Health Organization

In recent news, the International Agency for Research on Cancer issued a statement saying, “shift workers and firefighters have a higher risk of cancer than the general population.” They go on to state that “such work should be classified as probably or possibly carcinogenic.” These conclusions are based on years of published research. More studies are in the process of being done to further confirm the link but it is apparent that shift work disturbs the body’s internal clock, thus having cancer-causing effects.

This “internal clock,” also known as the circadian rhythm, is a cycle that tells our bodies when to sleep, wake up and eat, among other important physiological processes. Our internal clock can be affected by environmental cues like sunlight and temperature. When one's circadian rhythm is disrupted, such as in the case of those who do shift work, sleeping and eating patterns can be negatively affected. Disrupted circadian rhythm can also increase the chances of cardiovascular events, obesity, and a correlation with neurological problems like depression and bipolar disorder.

Dr. Kelly Halderman

Dr. Kelly Halderman has a Naturopathic medical degree from Kingdom College of Natural Health. She currently is working on completing her Ph.D. in clinical nutrition and has certification in Plant-Based Nutrition from Cornell University.
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How does this correlate with cancer? Several tests in mice have shown that genes that control our circadian clock are disrupted in tumor cells. The field studying chronobiology, which involves research on our circadian rhythm, is growing exponentially; researchers were awarded the 2017 Nobel Prize in physiology for their work in this area.

Nearly 20 percent of the working population in Europe and North America is engaged in shift work. Shift work is most prevalent in the health-care, industrial, transportation, communications, and hospitality sectors. In 2001, a team at the Fred Hutchison Cancer Research Center in Seattle found that women who work night shifts might have a 60 percent greater risk of breast cancer. A journal of the American Association for Cancer Research, Cancer Epidemiology, Biomarkers & Prevention recently published an analysis of 61 studies that included nearly four million people.

The researchers looked closely at the types of cancer women were getting — they found that women who worked night shifts for longer periods of time had a 41 percent higher risk of skin cancer, 32 percent higher risk of breast cancer and an 18 percent greater risk of digestive system cancers compared to women who did not work night shifts. “We are always going to have night workers and shift workers. Some jobs must be done around the clock like nurses. We need to know how to reduce the risk,” reports a representative for the IARC.

What can be done?

Since most people are not able to easily change occupations we can focus on dampening this risk by implementing what called Time Restricted Eating (TRE). TRE can minimize the ill effects of eating in larger, unnatural timeframes, as we see in those who do shift work. TRE has strong science behind the improvements in immune system function, which is essential in minimizing cancer risk. The gut is the foundation of health, giving it time to rest and repair is imperative.

The secret to TRE is to only eat in a window of time each day. Most experts believe the greatest health benefits of TRE are to limit one’s eating window to 10-12 hours. Eating outside a window over 10-12 hours puts enormous stress on not just the gastrointestinal system but also our inherent detoxification systems.

Without normal repair mechanisms being employed, our gut becomes “leaky” thus spurring on leakage of antigens and ramping up our immune system against in the gut causing excess, chronic inflammation. This has been shown to be a cornerstone of why the shift worker group has an increased risk for cancer.

The shift work findings may also be correlated with the body’s response to light. The pineal gland, found deep in the brain, produces the hormone melatonin after the body is exposed to sunlight (or artificial light) and then to darkness. Production of melatonin is disrupted when people are awake at night (or awoken even in short spurts) with the pineal gland being exposed to light when it is normally not meant to be.

Melatonin production is normally increased after dusk — its action is to get the body prepared and facilitate restful sleep. It attaches to receptors in both the brain and the pancreas, informing them to secrete hormones that help induce sleep. Melatonin also acts as an antioxidant protecting DNA from the type of damage that leads to diseases like cancer and heart disease. Therefore, when one has less secretion of melatonin, as in shift workers, sleep is disturbed and disease risk increases.

More research is being done to examine other factors that help explain the connection between increased cancer risk and shift work. We do know this demand for this type of work is not decreasing and may never be able to replace so efforts to help those so employed with lifestyle choices like the TRE are important. The World Health Organization’s categorization of shift work as a possible carcinogen should prompt us to continue to make changes to minimize this effect.

We at the Conners Clinic work to implement strategies to lessen the effect of this risk and treat those with cancer who need to implement such changes, help individuals through the process of detoxification, gut repair, or other therapies that can minimize the effects of their occupation.

Contact Dr. Halderman by visiting or calling (800) 209-4833.

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  1. Fangyi Gu, MD, ScD (2015) Total and Cause-Specific Mortality of U.S. Nurses Working Rotating Night Shifts. American Journal of Preventative Medicine. Volume 48, Issue 3, Pages 241–252
  2. Xia Yuan, (2018) Night Shift Work Increases the Risks of Multiple Primary Cancers in Women: A Systematic Review and Meta-analysis of 61 Articles. Cancer Epidemiology, Biomarkers and Prevention. Volume 27, Issue 1, Pages 25-40.
  3. Bulletin of the World Health Organization 2013;91:626-627. doi:
  4. Panda, Satchidananda, Ph.D. (2016). Fasting, circadian rhythms, and time restricted feeding in healthy lifespan. Cell Metab.Volume 23(6): 1048–1059.