Woman who have worked night shifts for at least 30 years have twice the risk of breast cancer, according to a new study.

Doctors believe that the risk is increased by a number of factors: a decrease in the hormone melatonin which reacts to light; sleep disturbance and lifestyle. It has also been suggested that the reduction in vitamin D due to the lack of sunlight could also contribute.

Researchers in Canada compared 1,134 women with breast cancer to 1,179 others of the same age who did not have the disease, in the study which is published in Occupational and Environmental Medicine.

About one-third of the women in both groups said they had worked night shifts. Those who had long term at night for 30 years or more were found to have twice the risk compared to those who did not.

Similar research among nurses has already established the link, but this is the first study amongst the general public.