Women who work night shifts for 30 years or more may be at an increased risk of developing breast cancer, a new study indicates.

Previous studies have suggested a potential link between shift work and the disease, however the findings have not been conclusive and much of this research has focused on nurses only, rather than the general population.

Canadian scientists set out to investigate whether night shifts had increased the risk of breast cancer in over 1,100 women with the disease and 1,100 without it.

The participants were the same age and had held a range of different jobs. They were asked about their working patterns over the years and their tumour type was determined using hospital records.

Around one-third of women from both groups had worked night shifts during their working lives.

The scientists found that women who worked night shifts for up to 14 years, or between 15 and 29 years, were not at an increased risk of developing breast cancer.

However, those who had worked night shifts for 30 years or more had double the risk of developing the disease. This was irrespective of whether the women worked as nurses or in other jobs.

"As shift work is necessary for many occupations, understanding which specific shift patterns increase breast cancer risk, and how night shift work influences the pathway to breast cancer, is needed for the development of healthy workplace policy," the scientists from Queen's University in Ontario said.

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