In response to increased demand for 24-hour services, one in five workers in more industrially developed countries now work outside normal office hours. This can play havoc with our biological clock (sleep, healing, etc.), leading to (e.g.) gastro-intestinal disorders and heart disease.

According to Shanta Rajaratnam and Professor Josephine Arendt from Surrey University’s Centre for Chronobiology,: “biological time ... greatly affects the productivity and health of a nation. The cost to the nation’s health of working out of phase with our biological clocks is probably incalculable at present. ... Employers and individuals need to be aware of the major performance and alertness decrements associated with night activity and how to best manage and counteract them ... Greater regulation of work practices during these times (i.e. outside the 9-5 working day) is warranted".

A study estimated the cost to the US in reduced performance and accidents at about $16 billion a year (Lancet 2001;358:999-1005).

Two new studies published in the Journal of the US National Cancer Institute (1.10.01) suggest 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. (In a study published last year, Danish scientists 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.)

The results of the latest study were based on the work history of 763 women with breast cancer and 741 without. Changes in melatonin levels in men doing nightshifts may increase the risk of some types of male cancer, according to Dr Davis.

The second study, an analysis by researchers at the Brigham and Women’s Hospital in Boston (US) of the results of 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 suspect that interrupted melatonin production (which occurs when the eye is exposed to light during what is supposed to be a sleeping period) is 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.- 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.

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