Many research papers have concluded that those on night shifts at supermarkets, bars, factories, hospitals and other venues may be putting themselves at heightened risk for cancer.

For the first time the International Agency for Research on Cancer (IARC) has added night shift work to the official list of "probable" carcinogens, having studied human epidemiological data, animal study results, and research looking at possible mechanisms linking night work to tumour 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 night-shift cancer link.

Category One risks are known carcinogens such as asbestos. Night working now sits just one rung below that: a probable cause of cancer. Dr Vincent Cogliano of the IARC said they reached their conclusion after looking at a wide number of studies of both humans and animals.

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," Aaron 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.

There are also investigations into possible biological mechanisms linking working through the small 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. 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.

Already the Danish government has paid compensation to several female workers who have suffered breast cancer.