Women working night shifts are at greater risk of breast cancer, according to new research.
Health on Female First
Working night shifts for 30 or more years doubles the risk of developing breast cancer, and is not confined to nurses as previous research has indicated, finds a study published online in Occupational and Environmental Medicine.
Shift work has been suggested as a risk factor for breast cancer, but there has been some doubt about the strength of the findings, largely because of issues around the assessment of exposure and the failure to capture the diversity of shift work patterns. Several previous studies have also been confined to nurses rather than the general population.
In this study, the researchers assessed whether night shifts were linked to an increased risk of breast cancer among 1134 women with breast cancer and 1179 women without the disease, but of the same age, in Vancouver, British Columbia, and Kingston, Ontario.
The women, who had done various different jobs, were asked about their shift work patterns over their entire work history; hospital records were used to determine tumour type.
This may be important, say the authors, because risk factors vary according to hormone sensitivity, and the sleep hormone melatonin, disruption to which has been implicated in higher breast cancer risk among night shift workers, may boost oestrogen production.
Dr Hannah Bridges, from leading charity Breakthrough Breast Cancer, says, “This is one of a few studies that suggest working night shifts for many years may increase breast cancer risk. However, we don’t yet know that shift work is a risk factor for breast cancer, so we’d urge women not to panic.
“We need to better understand why night work might increase breast cancer risk. Shift work may lead to unhealthy lifestyle habits that could independently increase the risk of breast cancer, so we’d encourage all women to take part in regular physical activity, maintain a healthy weight and reduce their alcohol intake.
“The Breakthrough Generations Study is following more than 115,000 UK women for 40 years in order to discover more about the causes of breast cancer. Part of the study is looking at shift workers to help us find out more about breast cancer risks for these women and to understand how risks could be minimised in the future.”
Around one in three women in both groups had worked night shifts. There was no evidence that those who had worked nights for up to 14 years or between 15 and 29 years had any increased risk of developing breast cancer.
But those who had worked nights for 30 or more years were twice as likely to have developed the disease, after taking account of potentially influential factors, although the numbers in this group were comparatively small.
The associations were similar among those who worked in healthcare and those who did not. Risk was also higher among those whose tumours were sensitive to oestrogen and progesterone.
The suggested link between breast cancer and shift work has been put down to melatonin, but sleep disturbances, upset body rhythms, vitamin D or lifestyle differences may also play their part, say the authors.
“As shift work is necessary for many occupations, understanding which specific shift patterns increase breast cancer risk, and how night shift work influences the pathway to breast cancer, is needed for the development of healthy workplace policy,” conclude the authors.
Tagged in Breast Cancer Health Women's Health