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According to new international research, working at night could increase a woman’s risk of breast cancer, especially in those who are pre-menopause.

Carried out by Canadian, Australian, and European researchers, the new study surveyed more than 13,000 women aged on average 55 to 59 from five different countries (Australia, Canada, France, Germany and Spain).

The participants included 6,093 women who had been diagnosed with breast cancer and 6,933 women without breast cancer who acted as controls.

The findings, which were published in the European Journal of Epidemiology, suggested that the risk of developing breast cancer increases with the number of hours worked per night, as well as the number of years spent on the night shift.

Women who worked at least three hours between midnight and 5 a.m. every night had a 12 percent greater risk of developing breast cancer than women who had never worked at night.

Among pre-menopausal women, this risk increases to 26 percent.

In addition, the team also found that pre-menopausal women who work night shifts longer than 10 hours showed a 36 percent increased risk of breast cancer compared to those who have never worked nights, with the risk as high as 80 percent among women who work night shifts longer than 10 hours for more than three nights per week.

However, the risk appeared to decrease two years after stopping night shift work, with women who were still working nights at the time of the study showing a breast cancer risk 26 percent higher than those who had stopped working at night at least two years previously.

Although the analysis of the five surveys did not specifically consider the type of night work, the Canadian survey did compare women working in healthcare and other sectors.

“The risk associated with breast cancer and night work varied little among respondents, regardless of job type,” explained study co-author Anne Grundy, from the Université de Montréal.

“Although we are not fully certain, we believe that this risk could be related to the hypothesis that night work disrupts circadian rhythm and inhibits the secretion of melatonin, which may protect against cancer,” she added.

The researchers also added that further research should be carried out to investigate the difference in risk between pre- and post- menopausal women. JB

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