Researchers found those who work late shifts three times a week for six years are at four times the risk of breast cancer.
A study of more than 18,500 women who worked for the Danish Army between 1964 and 1999 found the risk was linked to how often and how long they worked unsocial hours.
The findings revealed night shift work was associated with a 40 per cent increased risk of breast cancer compared with those who never did any.
Women who worked unsocial hours three times a week or more for at least six years were more than twice as likely to have the disease as those who had not. The risk quadrupled for those working this shift pattern for this length of time if they described themselves as “morning” types.