(GUARDIAN) — Working night shifts for more than 30 years could double women’s risk of developing breast cancer, a study has suggested. Canadian researchers examined 1,134 women with breast cancer and 1,179 women of the same age without the disease.

They found that nurses, cleaners, care workers, some shop workers, call centre handlers and others who work night shifts for a long term can have twice as high a risk of developing the disease than those who do not.

Women were asked about their work and shift patterns and researchers also assessed the hospital records of the women with the disease. About a third of the women had a history of night shift work.

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