Fresh research from Denmark, covering women who have worked in the country's military, and published on 28 May in the journal Occupational and Environmental Medicine, showed that night shift work was associated with a 40% increased risk of breast cancer.The study, carried out by the Danish Cancer Society's institute of epidemiology, shows that 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. But there was "a neutral link" for those who worked only one or two night shifts per week.

The study also challenged a hypothesis that less exposure to the sun and vitamin D might be a risk factor for those women who worked night shifts. The researchers found that in fact night workers tended to sunbathe more than those who worked during the day.

Night work can not only disrupt body clocks and result in sleep deprivation, however. It has been argued that it also suppresses production of the hormone melatonin and other metabolic and physiological processes that may increase the growth of tumours.The authors of the research set up a case-control study from more than 15,500 women who had been in the Danish military. Of those, 141 with breast cancer and 551 without filled in a 28-page questionnaire on work and lifestyle. But the analysis was only based on 132 cases and 505 controls.

In the UK, the Health and Safety Executive (HSE) has commissioned the cancer epidemiology unit at Oxford University to investigate the links between working night shifts and breast cancer in women in an attempt to establish whether working at night increases the risk of chronic disease. The study should be completed in 2015.

The World Health Organisation's International Agency for Research on Cancer said five years ago that working shifts that disrupted circadian rhythms was "probably" carcinogenic – a decision that first prompted Denmark to pay compensation to women claimants with breast cancer who had worked night shifts for longer than 20 years. The money has come through employers' insurance.

Source: The Guardian