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Cancer Rates Higher in Night Shift Workers, Leads WHO to List Graveyard Shift as 'Probable' Carcinogen

Nov 30, 2007 | Parker Waichman LLP

Working the overnight shift has been linked to an increased risk of developing cancer.   Though the association may seem a bit far fetched, research has found that men who work the overnight shift have higher rates of prostate cancer, while women have higher rates of breast cancer.   The overnight shift - cancer link is strong enough that the International Agency for Research on Cancer, the cancer arm of the World Health Organization (WHO), recently added the graveyard shift to its list of probable carcinogens.

The night shift - cancer link was first brought to light in 1987, when Richard Stevens, a cancer epidemiologist and professor at the University of Connecticut Health Center, published a paper suggesting the connection.  Stevens speculated that a sudden rise in breast cancer rates in the 1930s was somehow connected to the increased prevalence of night shift work brought on by industrialization.  Most other scientists, however, scoffed at Stevens' assertions. But over the years, more and more research has pointed to a connection between overnight work and cancer rates.  Also, lab animals that have their light and dark schedules switched have been shown to develop more tumors and die earlier. 

Now, scientists think there might be a real biological basis for the night shift - cancer link.  Working the night shift is dangerous because it disrupts the circadian rhythm - the "clock" that tells the body when to sleep and when to wake, and regulates other important biological processes.   The hormone melatonin, which is vital to the suppression of tumors, is produced at night.  Light shuts down melatonin production, so being exposed to artificial light in the evening could mean a melatonin deficiency. This could be one reason for the higher rates of cancer among night shift workers.

The balance between light and dark is very important to the body in other ways, and many of those processes could also play a role in determining a person's resistance to cancer.  Not getting enough sleep - a common problem for night shift workers who often have other daytime responsibilities - weakens the immune system, making the body more vulnerable to an attack from cancer cells.  Certain processes, like cell division and DNA repair can also be influenced by exposure to light and dark.

The WHO decision to call night shift work a probable carcinogen doesn't mean such work definitely causes cancer - just that the connection is plausible.  Still, if the link between cancer and overnight work is verified, it could have vast implications.  It would mean that millions of people who work the graveyard shift - including 20% of the people living in developing countries where healthcare is poor - are at a higher risk for cancer.

Not much is known about how frequency or length of time spent on the night shift affects cancer.  Most studies involving the night shift - cancer link have been done either on lab animals, or with small groups of workers like nurses and airline crews.  Much more work needs to be done to determine how serious the cancer risk is for people on the graveyard shift.  It is hoped that the WHOs designation of night shift work as a probable carcinogen will spark some of that much needed research.

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