(XINHUA) – Working night shifts for 30 or more years doubles the risk of developing breast cancer in a diverse mix of occupations, according to a new Canadian study.

The research, recently published in the journal Occupational and Environmental Medicine, showed that the link between long-term night shift work and increased breast cancer risk is not confined to nurses as previous research has indicated.

In the study, researchers from Queen’s University and British Columbia Cancer Agency studied 1134 women with breast cancer and 1179 women of the same age but without the disease.

The participants had come from various occupational backgrounds, and were asked about their shift work patterns over their entire work history. Hospital records were used to determine tumor type as well.

The researchers found 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.

The suggested link between breast cancer and shift work has been linked to melatonin. However, sleep disturbances, upset body rhythms, vitamin D or lifestyle differences may also play their part, according to the researchers.