Nov. 30, 2007 -- Working the night shift may cause cancer, according to a report published in The Lancet.

The report comes from a team of 44 scientists in 10 countries commissioned by the World Health Organization's International Agency on Cancer Research.

They report "limited" evidence of a connection between cancer and night shift work in people. That evidence included a higher rate of breast cancer in female nurses who work night shifts.

But those studies, which were observational, don't prove cause and effect. Many genetic and environmental factors affect cancer risk.

So the scientists also reviewed studies in which animals were exposed to light at night, disrupting the animals' so-called body clocks (circadian rhythm).

Those studies provided "sufficient evidence" of a connection between circadian rhythm disruption and cancer, states the report.

The scientists concluded that "shift work that involves circadian rhythm disruption is probably carcinogenic to humans," write Kurt Straif, MD, and colleagues.



They note that shift work may raise cancer risk by suppressing production of melatonin, a chemical involved in the circadian rhythm.

Straif works in Lyon, France, for IARC.

The scientists also concluded working as a painter is carcinogenic and that working as a firefighter is "possibly carcinogenic" due to on-the-job chemical exposure.