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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.”

The hormone affects many different physiological systems, Blair added. “It is also an antioxidant — a sink for chemicals that are normally dangerous to life,” he noted.

Melatonin can affect the immune system, as well, including cancer-suppressing genes, Blair said.

Night shift workers may also have to deal with disrupted sleep patterns, another expert pointed out. “Night shift people tend to be day shift people who are trying to stay awake at night,” Mark Rea, director of the Light Research Center at Rensselaer Polytechnic Institute in New York, told the Associated Press.

Altered sleep patterns and sleep deprivation weaken the immune system, he said, and upset natural rhythms the body uses to maintain healthy cells.

Blair stressed that the IARC has only defined night shift work as a “probable” cancer risk — there’s not enough proof to place it in the “definite” category alongside such villains as asbestos and smoking.

So, is there anything night shift workers can do to reduce their potential risk? Besides switching to a day job, maybe not a lot — experts don’t recommend long-term melatonin supplementation, because it may undermine the body’s ability to produce the hormone naturally.

“It appears that the impact of shift work is greatest if you keep changing the shift that you are on,” Blair said. If you find yourself working at night, then “it’s better that you are always a night shift worker,” he said. Switching back between day and night shifts is really tough on the body’s circadian clock, and “there was the sense that this might be the most hazardous type of shift work that you could be engaged in,” Blair said.

The American Cancer Society said it was reserving judgment on the new listing, noting that it had not yet reviewed the literature in depth.

“We understand that the epidemiologic literature is complex, the study results have not been entirely consistent, and that exposure itself is not easy to classify or measure,” Elizabeth Ward, director of surveillance research at the ACS, said in a statement. She stressed that the society does not itself create a list of carcinogens but instead relies on IARC and the U.S. National Toxicology Program to do so. - Yahoo