Working night shifts more than twice a week is associated with a 40 per cent increased risk of breast cancer, a study has found. The long term study, published online on 28 May 2012 in Occupational and Environmental Medicine, found those who had worked nights at least three times a week for at least six years were more than twice as likely to have the disease as those who had not.

Johnni Hansen and Christina F Lassen. Nested case-control study of night shift work and breast cancer risk among women in the Danish military, Occupational & Environmental Medicine, Online First, 28 May 2012, doi 10.1136/oemed-2011-100240.
TUC news release and occupational cancer guide [pdf]. Alliance for Cancer Prevention news release. The Guardian. Daily Mail. The Telegraph. Risks 558.