Two studies suggested a link between working night shifts and an increased risk of developing breast cancer:

• Scott Davis and colleagues at the Fred Hutchinson Cancer Research Centre in Seattle (US) found that women who regularly worked night shift for three years or less were about 40% more likely to develop the disease. Women who worked night shift for four years or more ran a 60% increased risk (Journal of the US National Cancer Institute 1.10.01). The study compared the work history of 763 women with breast cancer and 741 without. Scott also warned that changes in melatonin levels in men doing nightshifts might increase the risk of some types of male cancer

• The second study, an analysis of the data gathered by the US Nurses Health Study,* found an 8% increased risk in nurses who worked rotating night shifts at least three times a month for 1-29 years, and a 30-36% increased risk in those working to that pattern for more than 30 years (Hanson,J et al. Epidemiology 2001;12(1):74-77)

Both teams suspected that interrupted melatonin production (which occurs when the eye is exposed to light during what is supposed to be a sleeping period) was the cause. It is known that low melatonin levels increase the production of oestrogen and that excessive levels of oestrogen increase the risk of breast cancer.

Ed.- (i) The precise mechanism is unproven. Previous studies have implicated (a) disruption of night-time secretion of the hormone melatonin, and (b) high nocturnal exposure to electromagnetic fields.

(ii) A Danish study published in 2000 found that women who had worked predominantly at night for at least six months in their working life were 50% more likely to develop breast cancer.

* A study covering the health outcomes of 78,562 nurses 1988-1998

See also Night shifts lead to worse health and accidents