Night Shifts and Cancer

The World Health Organisation (WHO), in 2007, published a review which identified seven studies suggesting that sleep disruption may be carcinogenic to humans. A number of studies suggested an association between shift work and breast cancer. However, the University of Oxford, funded by the UK Health and Safety Executive, Cancer Research UK and the Medical Research Council, has recently found that ‘working night shifts has “little or no effect” on a woman’s risk of developing breast cancer.[i]

The review looked at data from 10 different countries and pooled the evidence of three large UK-based studies of post-menopausal women, The Million Women Study, EPIC-Oxford and the UK Biobank.

In all three studies, participants were asked about their employment and whether their job involved working night shifts. The answers were categorised into:

• never/rarely
• sometimes
• usually
• always
  

The participants were followed via records linked to the NHS Central Registers which provide information on cancer registrations and deaths. The outcomes of interest in this analysis were the first diagnosis of breast cancer or death from breast cancer.

Overall, it was found across the three studies, that there was no significant link between night shift work for any number of years and risk of breast cancer. Even when combining these results with the seven non-UK studies included in the previous 2007 WHO review, there was still no evidence that night shift work was associated with breast cancer.

The research did point out that the studies reviewed were all observational studies and so the possibility that other health and lifestyle factors associated with night-shift work, such as obesity or smoking, could increase breast cancer risk still can’t  be ruled out.

We review the evidence further in a future feature.