Frequent night shifts can be linked to an increased risk of breast cancer, research has argued.
A study of women in the Danish military, published in the journal Occupational and Environmental Medicine in May 2012, has concluded that working night shifts more than twice a week is associated with an increased risk of breast cancer.
The risk, the research said, also seems to be cumulative and strongest among those who describe themselves as "morning" people or "larks", rather than "evening" people or "owls".
The research highlighted that between 10% and 20% of the workforce in both Europe and the US works night shifts.
In 2007 the International Agency for Research on Cancer concluded that shift work, which disrupted the body clock (circadian rhythms), was "probably carcinogenic", but called for more research.
This latest study covered more than 18,500 women working for the Danish army between 1964 and 1999, all of whom had been born between 1929 and 1968.
From the 692 responses - of which 141 were from women with breast cancer - it was calculated that, overall, night-shift work was associated with a 40% increased risk of breast cancer compared with those who did not work night shifts.
However, women who worked night shifts at least three times a week, and for at least six years, were more than twice as likely to have the disease as those who had not, the survey found.
Brendan Barber, TUC general secretary, said: "We need urgent advice from the Health and Safety Executive and the Government so that employers can reduce the risk of female workers developing breast cancer - for example, by identifying safer shift patterns."