A study by the World Health Organization has found that night shift work increases the risk of cancer in humans. The study found that after prolonged exposure to night shift work, women are at a higher risk to develop breast and colon cancer. Men who work the night shift are more likely to experience prostate cancer.

"Shift work that interferes with regular nighttime sleep disrupts circadian rhythms, our body's natural clock," said Dr Erhard Haus of HealthPartners Research Foundation who chaired a subgroup of the study. "This impedes biologic function by suppressing the immune system, reducing melatonin production, and may damage genes leading to the production of abnormal cells."

Haus was part of a team of 24 scientists from 10 countries who met at the WHO’s International Agency for Research on Cancer (IARC) in Lyon, France, to assess the potential carcinogenicity of shift work, painting, and firefighting. The workgroup analyzed several epidemiologic studies and animal experiments. In addition to their findings on shift work, they determined that overall, occupational exposure as a painter is carcinogenic to humans. They also classified occupational exposure as a firefighter as possibly carcinogenic to humans.

A preliminary report of the IARC appeared in Lancet Oncology, December 7, 2007.

Between 15% and 20% of the working population in the United States and Europe are engaged in shift work, which is most prevalent in the health care, transportation, communication, leisure, and hospitality sectors.