It was once scientific heresy to suggest that smoking contributed to lung cancer. Now, another idea initially dismissed as nutty is gaining acceptance: the graveyard shift might increase your cancer risk.

Next month, the International Agency for Research on Cancer, the cancer arm of the World Health Organization, will classify shift work as a “probable” carcinogen.

That will put shift work in the same category as cancer-causing agents like anabolic steroids, ultraviolet radiation, and diesel engine exhaust.

If the shift work theory proves correct, millions of people worldwide could be affected. Experts estimate that nearly 20% of the working population in developed countries work night shifts.

It is a surprising twist for an idea that scientists first described as “wacky,” a cancer epidemiologist and professor at the University of Connecticut Health Center, Richard Stevens, said. In 1987, Dr. Stevens published a paper suggesting a link between light at night and breast cancer.

Back then, he was trying to figure out why breast cancer incidence suddenly shot up starting in the 1930s in industrialized societies, where nighttime work was considered a hallmark of progress. Most scientists were bewildered by his proposal.

But in recent years, several studies have found that women working at night for many years are indeed more prone to breast cancer, and that animals who have their light-dark schedules switched grow more cancerous tumors and die quicker.

Some research has also shown that men working at night may have a higher rate of prostate cancer.

Because these studies have been done mainly in nurses and airline crews, bigger studies in different populations are needed to confirm or disprove the findings.

The idea that shift work might increase your cancer risk is still viewed with skepticism by some, but many doubters will likely be won over when IARC publishes the results of its analysis, the result of an expert panel convened in October, in the December issue of the Lancet Oncology.

The American Cancer Society said it would most likely add shift work to its list of “known and probable carcinogens” when the IARC makes its reclassification. Up to now, the society has labeled it an “uncertain, controversial, or unproven effect.”

Experts acknowledge the evidence is limited, but the “probable” tag means that a link between shift work and cancer is plausible. “The indications are positive,” the director of the Monographs program at IARC, which decides on carcinogen classifications, Vincent Cogliano, said. “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.”

The research suggests a correlation between people who work at night and increased cancer rates. But the cause of the cancer might still be something else that people who work at night do that is unaccounted for in the research.

Scientists suspect that shift work is dangerous because it disrupts the circadian rhythm, the body’s biological clock. The hormone melatonin, which can suppress tumor development, is normally produced at night.

Light shuts down melatonin production, so people working in artificial light at night may have lower melatonin levels, which scientists think can raise their chances of developing cancer.

Sleep deprivation may also be a factor. 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,” Mark Rea, the director of the Light Research Center at Rensselaer Polytechnic Institute in New York, said. Mr. Rea is not connected to IARC or its expert panel.

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.