In the study, they assessed whether night shifts were linked to an increased risk of breast cancer among 1134 women with breast cancer and 1179 women without the disease, but of the same age, in Vancouver, British Columbia, and Kingston, Ontario.
The women, who had done various different jobs, were asked about their shift work patterns over their entire work history; hospital records were used to determine tumour type.
Around one in three women in both groups had worked night shifts. There was no evidence that those who had worked nights for up to 14 years or between 15 and 29 years had any increased risk of developing breast cancer.
But 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, however, the numbers in this group were comparatively small.
The authors said that the suggested link between breast cancer and shift work has been put down to melatonin, but sleep disturbances, upset body rhythms, vitamin D or lifestyle differences could also play their part.