Canadian researchers examined 1,134 women with breast cancer and 1,179 women without the disease, but of the same age. Women were questioned about their work and shift patterns and researchers also assessed hospital records of the women who suffered from the disease. About a third of the women had a history of night shift work.
The study, published in Occupational and Environmental Medicine, found that those who had worked nights for 30 or more years were twice as likely to have developed the disease, after taking account of potentially influential factors, although the numbers in this group were comparatively small.
No such relationship was found if women worked for less than 30 years doing shift work.