Shiftwork has been recognised officially as a “probable” cause of cancer. The International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organisation, has said it will classify overnight shift work as a probable carcinogen after evidence was considered by a meeting of experts; IARC experts also ranked occupational exposure as a painter as carcinogenic to humans and as a firefighter as possibly carcinogenic to humans.

Kurt Straif and others. Carcinogenicity of shift-work, painting, and fire-fighting, The Lancet Oncology, volume 8, number 12, pages 1065-1066, December 2007.