Women who work night shifts are at higher risk of breast cancer, warn researchers.

Their findings suggest working at night increases the chances of the disease by 40 per cent.

Women working more than two night shifts a week have double the risk of those on day shifts, says a report from scientists, while night workers who also describe themselves as ‘morning people’ or ‘larks’ have a stronger risk than those who say they are ‘night owls’.

Experts believe a hormone in the body that potentially suppresses tumours may be disrupted by constant exposure to light during night-time hours.

There has been mounting evidence that night shifts might boost cancer risk because of the disruption to the body clock and hormone production.

The latest study, backed by the Danish Cancer Society, involved more than 18,500 women working for the Danish army between 1964 and 1999.

Researchers were able to contact 210 women out of a total of 218 who had breast cancer between 1990 and 2003 and who were still alive in 2005/06.

These women were matched with 899 women of the same age who had also worked for the Danish army but had not developed breast cancer.

The women completed a detailed questionnaire which included questions on their working patterns, use of the Pill and HRT, sunbathing habits and whether they classified themselves as a ‘morning’ or ‘evening’ person.

Overall, night shift work was linked with a 40 per cent increased risk of breast cancer compared with no night shifts. But women who had worked night shifts at least three times a week for at least six years were more than twice as likely to have contracted the disease as those who had not.

Those working this shift pattern for this length of time were even more likely to develop breast cancer if they were ‘morning’ types, says a report in the journal Occupational and Environmental Medicine.

They were almost four times as likely to have the disease as those who worked no night shifts, possibly because they are more susceptible to body clock disruption.

Researcher Johnni Hansen said the findings suggested that working up to two nights a week was not long enough to disrupt the body clock.

But frequent night shifts for several years may disrupt circadian rhythms – the body clock – and sleep patterns.

Exposure to light at night inhibits production of melatonin, which is produced by the pineal gland in the brain between the hours of 9pm and 8am. Melatonin, a hormone which dictates the natural cycles that govern sleep patterns, helps suppress tumours.

Research suggests that unusually low levels of melatonin, which are seen in people exposed to light during the night, may promote tumour growth.

Dr Rachel Greig, of the charity Breakthrough Breast Cancer, said: ‘It may be that night shifts themselves are not the only cause, as shift work can increase the likelihood of other lifestyle risk factors, such as lack of exercise.

‘All women should cut back on alcohol, get regular physical activity and maintain a healthy diet to reduce their risk of breast cancer.’