Working night shifts more than twice a week is linked to increased risk of breast cancer, a Danish study has found.
Frequent night shifts for several years may disrupt circadian rhythms and curb production of the cancer protecting hormone melatonin, researchers from Copenhagen’s Institute of Cancer Epidemiology suggest.
The findings, which are based on women who worked in the army for over 30 years, build on earlier evidence from research involving nurses. The Danish researchers surveyed 140 women who survived breast cancer and 550 who had not developed the disease.
The results, published online in the journal Occupational and Environmental Medicine, showed overall that night shift work was associated with a 40% increased risk of breast cancer, compared with working no night shifts.
More specifically, women who worked night shifts at least three times a week – and for at least six years – were found to be more than twice as likely to have the disease as those who had not worked shifts.
However, working up to two night shifts a week had no impact on breast cancer risk. This may not be long enough to disrupt circadian rhythms, said the authors.
Rachel Greig, senior policy officer at the charity Breakthrough Breast Cancer, said the study supported existing evidence of a link between shift work and increased breast cancer risk.
“The exact reasons are still not known and it may be that night shifts themselves are not the only cause, as shift work can increase the likelihood of other lifestyle risk factors, such as lack of exercise,” she said.