Irregular sleeping patterns have been "unequivocally" shown to lead to cancer in tests on mice, a study suggests. The report, in Current Biology, lends weight to concerns about the damaging impact of shift work on health.
The researchers said women with a family risk of breast cancer should never work shifts, but cautioned that further tests in people were needed.
Mice prone to developing breast cancer had their body clock delayed by 12 hours every week for a year. Normally they had tumours after 50 weeks - but with regular disruption to their sleeping patterns, the tumours appeared eight weeks earlier.