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A Danish study has provided further evidence that cumulative night-shift work increases the risk of breast cancer, prompting calls for more advice for employers and further research on the issue.

The researchers conducted a case-control study among a cohort of 18,551 female employees of the Danish military to investigate the risk of breast cancer after night-shift work, and to explore the role of leisure-time sun exposure and diurnal preference (whether the subjects were morning or evening people).

In keeping with the findings of previous studies in this area, the results did provide evidence of a link – specifically for those working intense night shifts for longer duration (the risk for those who worked only one or two night shifts a week was neutral). The researchers also found that women with morning preference who worked on night shifts tended to have a higher risk than those with evening preference.

This study, the results of which were published on the Occupational and Environmental Medicine website yesterday (29 May), differed from previous work in this area, which focused primarily on one profession (nursing) and did not take into account such modifiers as sun exposure and diurnal preference.

According to HSE-commissioned research by Imperial College London in 2005, an estimated 54 per cent of occupational cancer registrations in women are attributed to shift work. Between 5 and 20 per cent of UK workers are engaged in work that involves night shifts.

Commenting on the latest findings, TUC general secretary Brendan Barber said: “This study confirms previous research, which has shown that shift work is now the second biggest cause of work-related cancer deaths after asbestos. We need urgent advice from the HSE and government so that employers can reduce the risk of female workers developing breast cancer – for example, by identifying safer shift patterns.”

Charity Breast Cancer Care said that while the comparison of morning people with ‘night-owls’ was interesting, “far more robust research is needed to be able to draw any clear conclusion that this, coupled with working frequent night-shifts over a period of time, may be a significant risk factor contributing to breast cancer.

“Breast cancer is a complex disease influenced by genetics and environmental factors, as well as lifestyle choices.”

The HSE, which has placed shift work in third place (after asbestos and respirable silica) on its list of top ten agents/occupations attributed to causing occupational cancer, has commissioned the Cancer Epidemiology Unit at Oxford University to undertake an extensive study on the relationship between shift work and chronic disease.

The work will focus on shift-working patterns and lifestyle behaviours in relation to cancer and other chronic conditions in men and women, and is likely to be completed by the end of 2015. In the meantime, the Executive says its current guidance, HSG 256 Managing shift work, remains appropriate to help employers meet current legal obligations relating to hours of work and how they are scheduled.

The HSE is unlikely to undertake any proactive work on shift work before the results of the Oxford University research are delivered.