(Agencies)
Updated: 2007-11-30 09:26

Among the long list of agents that are listed as "known" carcinogens are alcoholic beverages and birth control pills. Such lists say nothing about exposure amount or length of time or how likely they are to cause cancer. The American Cancer Society Web site notes that carcinogens do not cause cancer at all times.

Still, many doubters of the night shift link may be won over by the IARC's analysis to be published in the December issue of the journal Lancet Oncology.

"The indications are positive," said Vincent Cogliano, who heads up the agency's carcinogen classifications unit. "There was enough of a pattern in people who do shift work to recognize that there's an increase in cancer, but we can't rule out the possibility of other factors."

Scientists believe having lower melatonin levels can raise the risk of developing cancer. Light shuts down melatonin production, so people working in artificial light at night may have lower melatonin levels.

Melatonin can be taken as a supplement, but experts don't recommend it long-term, since that could ruin the body's ability to produce it naturally.

Sleep deprivation may be another factor in cancer risk. People who work at night are not usually able to completely reverse their day and night cycles.

"Night shift people tend to be day shift people who are trying to stay awake at night," said Mark Rea, director of the Light Research Center at Rensselaer Polytechnic Institute in New York, who is not connected with the IARC analysis.

Not getting enough sleep makes your immune system vulnerable to attack, and less able to fight off potentially cancerous cells.

Confusing your body's natural rhythm can also lead to a breakdown of other essential tasks. "Timing is very important," Rea said. Certain processes like cell division and DNA repair happen at regular times.

Even worse than working an overnight shift is flipping between daytime and overnight work.

"The problem is re-setting your body's clock," said Aaron Blair, of the United States' National Cancer Institute, who chaired IARC's recent meeting on shift work. "If you worked at night and stayed on it, that would be less disruptive than constantly changing shifts."

Anyone whose light and dark schedule is often disrupted — including frequent long-haul travelers or insomniacs — could theoretically face the same increased cancer risk, Stevens said.

He advises workers to sleep in a darkened room once they get off work. "The balance between light and dark is very important for your body. Just get a dark night's sleep."

Meanwhile, scientists are trying to come up with ways to reduce night workers' cancer risk. And some companies are experimenting with different lighting, seeking a type that doesn't affect melatonin production.

So far, the color that seems to have the least effect on melatonin is one that few people would enjoy working under: red.

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