Working the night shift may cause cancer

| July 16, 2013

All the potential hazards in the workplace, artificial light could be one of the most frightening for those working the night shift.

Alongside other environmental factors like diesel exhaust particulates, “shiftwork involving light at night” was nominated last month to be formally reviewed for potential inclusion on the National Toxicology Program’s Report on Carcinogens. This public health report “identifies agents, substances, mixtures or exposures in our environment that may potentially put people in the United States at increased risk for cancer.”

The nomination proposes examining how shift workers’ nighttime exposure to artificial light – and the resulting disruption in circadian rhythms – might increase cancer risk. Circadian rhythms are the schedule of changes that take place in the body –  including sleep-wake cycles, the release of hormones and variations in body temperature – which are largely affected by light, according to the National Institute of General Medical Sciences.

If approved, this research will be just the latest on the subject.

Already in 2007, the International Agency for Research on Cancer, which is part of the World Health Organization, named shift work that disrupts circadian rhythms a “probable carcinogen” because it was found to increase breast cancer risks in women.

A study released this April also suggested a link between the night shift and an increased risk of ovarian cancer. A Canadian study published in the July issue of the journal Occupational and Environmental Medicine reported that women who worked the night shift in a variety of occupations for 30 years or more faced an increased risk of breast cancer.

cancer hazard sign

Will workplaces soon need a sign like this for artificial light at night? (Sign via

But the potential link isn’t restricted to women. Most of the research links the constant exposure to light with the body’s reduced production of melatonin, which increases the incidence and growth of tumors. Other studies have connected this to increased prostate cancer rates.

Just like other environmental carcinogens, the factors involved in the exposure will have to be broken down. Researchers will have to decide whether to investigate the effects of light at night, shift work or circadian disruption.

Within these factors, there are yet more variables that will have to be isolated. Several past studies that focused on how nurses are affected by night-shift work have been questioned because of the many other potential carcinogens faced in the occupation. Some researchers have suggested that studies need to extend beyond work and include heavy nighttime exposure to artificial light during leisure activities such as nighttime TV-watching or web-browsing.

photo of girl and light from computer

Research into the effects of artificial light at night could extend beyond the workplace to heavy exposure to TV or computer screens. (Photo by fundocter, via Creative Commons.)

Then there is the question of personal “diurnal preference.” A Danish study of military women working night shifts, published last year, found that those who considered themselves “early birds” had a higher breast cancer risk than their night-owl counterparts.

Yet another twist? The Lighting Innovations Institute at John Carroll University has reported that the melatonin-suppressing effect depends on the specific type of artificial light. The researchers say it’s limited to blue rays, which can be avoided by using a different type of light bulb or wearing “blue-blocking glasses.”

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