Women who work night shifts for long periods face a higher risk of breast cancer compared with those who work only in the daytime, experts say in a respected medical journal.

Researchers say that “circadian disruption” of sleeping patterns is “probably carcinogenic” while firefighters and painters are also exposed to cancer hazards through their work.

The report is issued by a working group of the International Agency for Research on Cancer, a unit of the World Health Organisation. Their assessment, based on a review of published epidemiological studies, will appear in this month’s issue of Lancet Oncology.

The results are based on the incidence of breast cancers among nurses and flight attendants, backed by animal studies which show that constant light, dim light at night or simulated jet lag substantially boost tumour development.

One of the authors, Vincent Cogliano, said the studies among working women showed that shiftwork posed a less than twofold increase in risk. “It is a real risk but one that epidemiologists would describe as a modest one,” he said.

Dr Cogliano said the risk for other professions and for other cancers was unclear but noted that nearly 20 per cent of the working population in Europe and North America was engaged in shiftwork of some kind.

One possible cause for the increased risk is that the circadian system is disrupted by exposure to light at night. This alters sleep activity patterns, suppresses production of the hormone melatonin and disregulates genes that control tumour development.

Among painters, occupational exposure to chemical solvents, pigments, additives and fine dust or asbestos is linked to “small but significant” increases in the risk of lung and bladder cancers, the IARC said.

Among firefighters, occupational exposure to chemicals, smoke and particles is considered “possibly carcinogenic”, meaning that there was limited evidence of carcinogenicity in humans and less than sufficient evidence of this in laboratory animals.

Explore Health

• Expert Advice

• Health Features

• Mental Health

• Alternative Medicine

• Child Health

• Health Club