Breast cancer is the most common form of cancer among South African women and one in every 29 women are expected to be affected in her lifetime.
The association between nightshift work and breast cancer was mainly observed in women working during overnight shifts, who worked at night for four and a half years or more, and less than three nights per week on average, said the researchers at the French health research body, INSERM.
"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," said INSERM director Pascal Guenel.
The link between nightshift work and breast cancer is believed to be caused by disruption of "body clock" genes, internal desynchronisation and sleep deprivation altering the immune system. However, the researchers said more research was needed to determine the exact causes.
The study was conducted in France among 1 232 women diagnosed with breast cancer between 2005 and 2007.
Sources: Cansa, Sapa, AFP