Women who work night shifts may have an increased risk of developing ovarian cancer, a new study indicates.

Around 330 new cases of ovarian cancer are diagnosed in Ireland every year. The disease can be difficult to detect because symptoms can be similar to more common conditions, such as bladder and digestive disorders. As a result, many women may present when the disease is already quite advanced.

Shift work that disrupts our circadian rhythm - the body's internal clock - is already recognised as a potential cancer-causing agent by the International Agency for Research on Cancer (IARC)

US scientists decided to look at this issue more closely. They followed the progress of over 1,100 women with the most common type of advanced ovarian cancer, almost 400 women with borderline ovarian cancer and more than 1,800 women not affected by the disease.

All of the women were aged between 35 and 74 and all were asked about the hours they worked. A similar number of women with ovarian cancer - 26% - were found to have worked night shifts compared to those without the disease - 22%.

The women worked these shifts for an average of 2.7 to 3.5 years.

The scientists found that overall, the use of the oral contraceptive pill was lower among those with ovarian cancer. They also tended to have fewer children. Being on the pill and having children are already known to reduce the risk of the disease.

However, they also found that working night shifts increased the risk of developing advanced ovarian cancer by 24% and the risk of early stage disease by 49%, compared to women who worked normal daytime hours.

The study noted that the results related only to women over the age of 50.

It also noted that there was no cumulative risk associated with the disease. In other words, the risk did not increase the more a woman worked these kinds of shifts.

The team from the Fred Hutchinson Cancer Research Center in Seattle said that one reason for this could be down to the hormone, melatonin. It is responsible for regulating reproductive hormones, especially oestrogen. However, it is normally produced at night and is suppressed by ambient light.

Details of these findings are published in the journal, Occupational and Environmental Medicine.