Working night shifts has little or no effect on a woman's risk of developing breast cancer, new research has found.

A review carried out in 2007 by the International Agency for Research on Cancer (IARC) stated that shift work disrupts the ‘body clock' and is a probable cause of cancer.

However, at that time, there was limited evidence in relation to breast cancer risk in humans, so that assertion was based on laboratory and animal studies. UK researchers decided to look into this further.

They looked at whether night shifts increase the risk of breast cancer by following the progress of 1.4 million women taking part in 10 different studies in a number of countries, including the UK, Sweden, the US and China.

They found that women who had worked night shifts, including those who had been doing this for 20 or 30 years, had no increased risk of developing breast cancer compared to those who had never worked night shifts.

In fact, the incidence of breast cancer was basically the same whether someone did no night shift work at all or had been working nights for many decades.

"This study is the largest of its kind and has found no link between breast cancer and working night shifts. Research over the past years suggesting there was a link has made big headlines, and we hope that this news reassures women who work night shifts," commented Sarah Williams of Cancer Research UK, which helped to fund the research.

Breast cancer affects one in nine Irish women, with around 2,800 new cases diagnosed every year. Women can reduce their risk by maintaining a healthy weight, being physically active and drinking less alcohol.

Details of these findings are published in the Journal of the National Cancer Institute.