When applying different previously used exposure metrics for night work.We found that estimates showed a slight(10%-30%) but nonsignificant increase of risk for breast cancer exposure categories.
By using consecutive night shifts as the exposure parameter,we observed significantly elevated breast cancer risks in the range of 1.7-1.8 in nurses who worked 5 or more years in jobs including ≥6 consecutive night shifts, compared with nurses who never worked night shifts.To our knowledge ,this is the first study to reveal an association between a higher number of consecutive night shifts and breast cancer risk.