The Danish government has become the first to begin paying compensation to women who developed breast cancer after working night shifts.
So far, according to BBC News, 40 women have been compensated by the Danish government. Those women who had a family history of breast cancer had their claims rejected.
Denmark's decision follows a 2007 ruling by a United Nations agency that night shifts probably increase the risk of developing cancer.
The International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization, reported that they had reviewed the literature on the effects of working the night shift and decided that it is "probably carcinogenic" to humans.
The IARC ranks cancer risks, placing such known carcinogens as asbestos into Category One: known risks. They placed working the night shift one rung below that: a "probable" cause of cancer.
The IARC reviewed dozens of studies linking night work and breast cancer. One of the studies, published in the Journal of the National Cancer Institute, showed a 36 per cent greater risk of breast cancer for women who had worked night shifts for more than 30 years, compared with women who had never worked nights.
They also cited more than 20 studies on lab rats that found an increased tumour rate among rats that were exposed to constant light, dim light at night, and simulated chronic jet lag.
The IARC experts noted that the higher cancer rates don't prove working overnight can cause cancer, pointing out that the link with shift work observed so far has been modest.
But their decision came because there was sufficient evidence in lab animals along with the limited evidence of cancers in humans to make the link.
Scientists suspect overnight work is hazardous to our health because it disrupts the body's circadian rhythm - its internal clock. It also disrupts the production of the melatonin, ae hormone that has been shown to suppress tumour development, and is produced primarily at night.
As further evidence of the circadian rhythm disruption theory, the IARC report also noted that studies have found the incidence of breast cancer is modestly increased in female flight attendants, who experience circadian disruption by frequently crossing time zones.
It's estimated that 15-20 per cent of the working population in Europe and the North America works shift work that involves work overnight.
Those most likely to do night shifts are those in the health-care industry, hospitality sectors, industrial manufacturing, transportation, news media and communications, and security workers.
