Those who work the graveyard shift for long periods or who experience jet lag on a regular basis face a higher risk of cancer, according to a World Health Organization cancer report.
A working group of the International Agency for Research on Cancer (IARC), the cancer arm of the WHO, says they have reviewed the literature on the effects of working the night shift and have decided that it disrupts the body's internal clock -its circadian rhythm -- and "is probably carcinogenic to humans."
The experts' assessment will appear in the December issue of The Lancet Oncology.
The experts point to studies of nurses who worked night shifts over the long-term. Those studies found that the nurses had a modestly higher risk of breast cancer compared with those who not engaged in night work.
They also noted that studies have found the incidence of breast cancer is modestly increased in female flight attendants, who also experience circadian disruption by frequently crossing time zones.
What's more, the experts note, more than 20 studies on lab rats have found an increased tumour rate among rats that were exposed to constant light, dim light at night, and simulated chronic jet lag.
The higher cancer rates don't prove that working overnight can cause cancer and the experts note that the link with shift work observed so far has been modest.
But the IARC's decision to designate night work as "probably carcinogenic" means there is limited evidence of cancers in humans but sufficient evidence of cancers in lab animals to make the link.
Scientists suspect that overnight work is dangerous because it disrupts both the circadian rhythm and the production of the hormone melatonin. Melatonin can suppress tumour development, but is produced by the body primarily at night.
The health alert affects millions around the world. IARC notes that about 15-20 per cent of the working population in Europe and the North America work 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, communication, and security workers.
Along with night shift workers, the IARC says painters and firefighters are also at risk of cancer.
Among painters, exposure to chemical solvents, pigments, or asbestos is linked to "small but significant" increases in the risk of lung cancer and bladder cancer, IARC said.
For firefighters, occupational exposure from chemicals, smoke and particles is considered "possibly carcinogenic."
The paper is published under the IARC's "Monographs Programme," in which the agency identifies environmental factors that boost the risk of cancer, thus providing policymakers and regulators with scientific evidence to support decisions on health and safety.
