Working night shifts does not lead to an increased risk of developing breast cancer, according to a new study by British researchers.

Since the 70s, scientists have suggested that increased exposure to electric light during night shifts could increase cancer risk by interfering with body’s internal clock, suppressing the sleep hormone melatonin and boosting estrogen. In 2007, the International Agency for Research on Cancer (IARC) designated sleep cycle disruption from night shifts as “probably carcinogenic,” according to The Telegraph.

But the new study, funded by the charity Breast Cancer Now, found no link between night shifts and breast cancer, in 102,869 women in the UK, over a ten-year period. The IARC is expected to review its own findings on night shifts later this year.

“A possible link between exposure to electric light at night and an increased risk of breast cancer was first proposed more than 30 years ago, but research has so far been inconclusive,” according to study co-author Michael Jones, staff scientist in genetics and epidemiology at London’s Institute of Cancer Research (ICR).

“In our new study we found no overall link between women having done night shift work in the last 10 years and their risk of breast cancer, regardless of the different types of work they did involving night shifts, and the age at which they started such work.”

The research was published in the British Journal of Cancer.

“Although night shifts may have other effects on people’s health, and we still don’t know the effect of a person’s body clock being disturbed for very long periods of time, it is reassuring to see more evidence suggesting that night shifts are not linked with a higher risk of breast cancer,” Jones said.

Based on the designation by IARC in 2007, Denmark made payments to dozens of night workers who attributed their breast cancer to having worked night shifts.

The study builds on the work of a 2016 meta-analysis that reached similar conclusions, but was challenged due to lack of details on the nature of the work, and an older average age for the women in the study.

The median age of the participants in the new study was 45. Overall, 2,059 out of the 102,869 participants were diagnosed with invasive breast cancer. With other risk factors taken into account, the researchers found no connection to their night shift work.

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