Night Shift Work May Heighten Risk for Cancer

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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.

More information

There's more on melatonin at the U.S. National Institutes of Health.