Working night shifts for more than 30 years could double women's risk of developing breast cancer, research suggests.
Nurses, cleaners, care workers, some shop workers, call-centre handlers and others who work night shifts for a long time can have a twice as high risk of developing the disease than those who don't, the new study found.
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 the hospital records for the women who suffered from the disease.
About one-third of the women had a history of night-shift work.
The study was published in Occupational and Environmental Medicine.
No such relationship was found if women worked for less than 30 years doing shift work. Previous research has linked the disease with shift work done by nurses but the latest study found that the associations were similar among those who worked in healthcare and those who did not.
"An association between more than 30 years of night shift work in diverse occupations and breast cancer is supported here, consistent with other studies among nurses," the authors of the study said.
"As shift work is necessary for many occupations, understanding of 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."
Experts cautioned that the increased cancer risk was yet to be confirmed.