Night work may increase a woman's chances of developing breast cancer by 30 percent -- a slightly elevated but "statistically significant" risk, French researchers said Tuesday.
This placed night work in the same order of risk as factors like genetic mutation, a late first pregnancy or hormonal treatment, Pascal Guenel, director of French health research body INSERM, told AFP.
Put into context, a smoker was eight times as likely to contract lung cancer as a woman working night shifts was to get breast cancer, he explained.
About 1,3 million women around the world are diagnosed with breast cancer every year.
In a study published in the International Journal of Cancer, the INSERM-led team said an association between night work with breast cancer "was mainly observed in women working during overnight shifts, those who worked at night for 4.5 or more years and less than three nights per week on average.
"The association was stronger in women who worked at night before their first full-term pregnancy than in women who started working at night later in life."
The scientists said more study was needed to determine the reasons.
Hypotheses include disruption of "body clock" genes, internal desynchronisation and sleep deprivation altering the immune system.
The study was conducted in France among 1,232 women diagnosed with breast cancer between 2005 and 2007.
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