The research, published in the British Journal of Cancer, involved researchers looking at data from 102,869 women in the UK over 10 years, 2,059 of whom later developed invasive breast cancer – the most common form of the disease.

Researchers from the Institute of Cancer Research (ICR) in London said that they found no overall link between night shift work and the likelihood of developing breast cancer.

The link between night shift work and an increased risk of breast cancer was first suggested in the 1970s.

Several studies have proposed that the exposure to electric light may increase the risk of developing the disease, by potentially disrupting the body's internal clock, suppressing levels of the sleep hormone melatonin and raising oestrogen levels.

In 2007, the International Agency for Research on Cancer said that shift work which disrupts the sleep cycle is "probably carcinogenic".

However, the new study’s lead researcher Dr Michael Jones says that previous research suggesting this has “so far been inconclusive”.

He added: "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.”

Jones said that "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 researchers hope their findings will help to reassure workers following decades of debate over the link between night shifts and breast cancer.

Baroness Delyth Morgan, chief executive at Breast Cancer Care and Breast Cancer Now which funded the research, 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."

The causes of breast cancer aren't fully understood. However the NHS states that the risk of developing the disease can be as a result of an increases in age, a family history of the condition, previous breast cancer or limps, and dense breast tissue.

Morgan added: "Whilst there are some things we can't change, there are steps all women can take to lower their breast cancer risk, such as maintaining a healthy weight, keeping physically active and drinking less alcohol.

"Even small changes are a great start."

The International Agency for Research on Cancer, which forms part of the World Health Organisation (WHO), will review evidence on night shift work and cancer in the summer of 2019.

The new study contradicts researchers from China last year who predicted that the risk of breast cancer increased by about 3.3 per cent every five years for workers who did clocked in for regularly night shifts.

Those involved in the study, published in the journal Cancer Epidemiology, Biomarkers & Prevention, came to the conclusion after analysing 61 studies which examined a link between long-term night shifts and 11 different cancers.

Overall they found that women who worked nights were 20 per cent more likely to have a diagnosis at some point.

The Independent

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