Night Shift Work May Heighten Risk for Cancer

By E.J. Mundell
HealthDay Reporter
Thursday, December 6, 2007; 12:00 AM

Copyright © 2007 ScoutNews, LLC. All rights reserved.

WEDNESDAY, Dec. 5 (HealthDay News) -- Workers on the night shifts at bars, convenience stores, hospitals and other venues may be putting themselves at heightened risk for cancer.

That's the conclusion of an international group of experts who plan to add night shift work to the official list of "probable" carcinogens.

A team of scientists at the International Agency for Research on Cancer (IARC) pored over human epidemiological data, animal study results, and studies looking at possible mechanisms linking night work to tumor formation.

"All three of those things suggested that, yes, this might be something that could contribute to human cancer," said Aaron Blair, scientist emeritus at the U.S. National Cancer Institute and chairman of the IARC Working Group that evaluated the shift work-cancer link.

The IARC -- a branch of the World Health Organization -- was expected to publish its findings in the December issue of The Lancet Oncology.

Although numerous studies have suggested a link between night shift work and cancer, this is the first time it has been evaluated by the IARC, Blair said.

On the epidemiological side, "there's human data -- nurses, airplane flight attendants, different groups that engage in shift work -- that have an elevated risk of breast cancer, and that's the strongest finding," Blair said. "There's lesser evidence, but some positive evidence, for [increased risk of] prostate cancer, and a little less, but still positive, evidence, for colon cancer," he noted.

In animal studies, rats exposed to light during their nocturnal, active phase, also displayed spikes in cancer incidence, Blair said.

Then there are investigations into possible biological mechanisms linking working through the wee hours to heightened odds for malignancy. The strongest theory involves melatonin, a hormone produced by the brain's pineal gland.

"Melatonin gets made during the dark period," Blair explained. "If you get light exposure during the normal dark period, it severely reduces the amount of melatonin that is made."