The Danish government has begun paying compensation to women who have developed breast cancer after long spells working nights.
Scientists suspect that overnight 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.
According to the journal The Lancet based on "the limited
evidence in humans for the carcinogenicity of shift work that involves night work," and "sufficient
evidence in experimental animals for the carcinogenicity of light during the daily work period (biological night), it has been concluded that "shift work that involves circadian disruption is probably carcinogenic to humans."
This is the first time though some compensation is being offered for cancer resulting from nightshifts.
Ulla Mahnkopf is one of those who has been compensated.
She spent 30 years as a flight attendant for the big Scandinavian airline SAS.
Long hours and disturbed working patterns came with the territory.
Then she developed breast cancer.
"It was awful telling my kids that, telling them what we were facing," she told me.
"It's not just the surgery but all the thoughts - do I survive this? I had bilateral cancer so not just one breast, two breasts."
At first she did not make the connection between her cancer and night working.
She said: "I had no idea.
"But when you think back now I can see that when I stopped flying it was like coming out of a shell, I had been living in there because of jet lag and I can see now I had a totally different life."
So far almost 40 Danish women have won compensation.
Not every case was successful: women who had a family history of breast cancer were among the ones whose claims were rejected.
The Danish authorities acted following a finding by the International Agency for Research on Cancer (IARC), an arm of the UN's World Health Organisation.
The IARC studies and ranks cancer risks.
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.
He said there was evidence to support the hypothesis that alterations in sleep patterns could suppress the production of melatonin in the body.
"Melatonin has some beneficial effects in preventing some of the steps leading to cancer," he said.
"The level of evidence is really no different than it might be for an industrial chemical."
In the UK unions estimate about 20% of the national workforce is involved in night shifts, BBC reported.
Professor Andrew Watterson, an occupational health specialist at Stirling University, said we are far behind Scandinavia in recognising the dangers.
"I think we can say there is a big public health problem here," he said.
"The evidence has been good over a long period of time about cardiovascular disease and night work, gastro-intestinal problems and nights
Work indicates there may be risks in terms of low birth-weight babies and longer pregnancies for women.
"We don't tend to identify the damage being done where shift working is prevalent and I think that's an error. The damage is there but we don't see it and we don't count it."
At the Health and Safety Executive, chief medical officer Dr John Osmond said they were aware of the debate and have commissioned their own research.
"The HSE has been very on the ball in this area and has commissioned a very eminent epidemiologist to examine the risk of working at night and whether there is any link to breast cancer. This report will be completed in 2011."