For more than 30 years, scientists have believed that working night shift increases the risk of developing breast cancer, but now a new long-term study has debunked that theory.

In 2007, the International Agency for Research on Cancer found that shift work disrupted the body’s sleep-wake cycle and was ‘probably carcinogenic’.

However, the evidence was always inconclusive, and now a new study by the Institute for Cancer Research in London has found that working night shift has no impact on breast cancer risk after all.

More than 100,000 women studied over a decade

The study was part of the Breast Cancer Now Generations Study, which is following more than 113,000 women over 40 years to examine the causes of breast cancer.

For this part of the study, researchers examined data from 102,869 women from the BCNG study over 9.5 years to identify who developed breast cancer.

Detailed information was collected about job type, the age at which women started and ended shift work, what nights they worked per week, average hours worked per night, and whether night shift was started before their first pregnancy.

The median age of participants at recruitment was 45 years, and 17.5% of participants had worked night shifts within the last ten years.

Data was also gathered on each participant’s known breast cancer risk factors, such as obesity, levels of physical activity, alcohol consumption, family history, age at first period and menopause, HRT use, number of children and age at their births, and duration of breastfeeding.

Upon conclusion of the study, data was gathered once again about night shift worked during the study period.

The study found no link between working night shift and the likelihood of developing breast cancer.

“We hope the findings will reassure women working night shifts”

Baroness Delyth Morgan, Chief Executive at Breast Cancer Care and Breast Cancer Now, which funded the study, said, “We hope these findings will help reassure the hundreds of thousands of women working night shifts that it’s unlikely their job patterns are increasing their risk of breast cancer.

“This question has been widely debated in recent decades and has understandably caused concern, and it’s encouraging that the evidence now suggests night shift work has no impact on breast cancer risk.”

Breast cancer risk is affected by a combination of our genes, lifestyle choices and events throughout life, Baroness Morgan said, and there is never one single cause of the disease.

To lower their breast cancer risk, women should be encouraged to have a healthy diet, maintain a healthy weight, keeping physically active, and drink less alcohol, Baroness Morgan said.

“We found no link between women having done night shift and their risk of breast cancer”

Dr Michael Jones, Staff Scientist in Genetics and Epidemiology at the ICR, said, “In our new study, we found no overall link between women having done night shift work in the last ten 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.

“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.”

The findings are set to be reviewed by the International Agency for Research on Cancer in the next few months.

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