Long term night shifts linked to doubling of breast cancer risk

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.

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.

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Screening tool helps identify health and safety impacts of shift work on individual workers

An international team of sleep researchers has developed the world’s first screening tool to help reduce workplace accidents and illnesses, including cardiovascular disease and cancer, caused by shift work.

Published in the journal Sleep, the new tool will enable health professionals and industry to better understand individual vulnerability to the health and safety impacts of shift work.

This screening questionnaire for a condition known as shift work disorder (SWD) has been developed by researchers from Monash University, and US partners, Harvard Medical School, Brigham and Women's Hospital, and the Henry Ford Hospital.

At least 15 per cent of workers in Australia, the US, and the United Kingdom, and around 23 per cent of workers in Japan are estimated to work outside normal hours, causing significant disruption to their natural sleep-wake schedules. SWD, characterised by extreme sleepiness and/or insomnia, is thought to affect around 10 per cent of shift workers.

Associate Professor Shantha Rajaratnam, of Monash and Harvard University, said the prevalence of shift work has been unknown due to the lack of accurate assessment tools.

"Shift work is a reality of modern economies, but research has shown that there are very real health risks associated with working outside regular hours," Associate Professor Rajaratnam said.

"Aside from associated health problems, shift workers are significantly more at risk of workplace injuries. The workers most affected by sleep disruption - those with SWD - account for a significant proportion of this risk and need to be identified."

Shift work, especially overnight, is associated with a higher rate of car crashes, industrial accidents, actual and near-miss injuries and quality-control errors on the job.

Secondary health problems linked with shift work include cancer, cardiovascular disease, diabetes, gastrointestinal diseases and mood disorders, including depression.

"This questionnaire is an important step in better understanding causes of vulnerability to shift work, and targeting interventions to those who most need them," Associate Professor Rajaratnam said.

"However, this is only a first step and further tests of actual impairment from lack of sleep must be developed for implementation in occupational settings."

"More collaboration between researchers, industry and government partners is needed to tackle these significant challenges and make shift work as safe and productive as possible."

Photo: freedigitalphotos.net

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