The research comes after a review in 2007 suggested shift work which disrupted the body clock was a probable cause of cancer.
Sarah Williams, Cancer Research UK’s health information manager, said of the new study: “Research over the past years suggesting there was a link has made big headlines, and we hope that today’s news reassures women who work night shifts.”
The new research, published in the Journal of the National Cancer Institute, examined whether night shift work increased women’s breast cancer risk by following 1.4 million women in 10 studies to see if they developed breast cancer.
Compared with women who had never worked night shifts, those who had ever done night work – including those who had worked nights for 20 or 30 years – had no increased risk of breast cancer.
“We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer,” said Dr Ruth Travis, lead author and a Cancer Research UK-funded scientist at the Cancer Epidemiology Unit at the University of Oxford.
Each year in the UK, around 53,300 women are diagnosed with breast cancer and roughly 11,500 die from the disease.
A previous review by the International Agency for Research on Cancer (IARC) classified shift work which disrupted the body clock as a probable cause of the cancer.
However at the time, there was limited evidence about breast cancer risk in humans so the classification was mainly based on a combination of animal and lab studies.
In the new study, researchers found that the incidence of breast cancer was the same whether someone did no night shift work at all or did night shift work for several decades.
The combined relative risks were 0.99 for any night shift work, 1.01 for 20 or more years of night shift work, and 1.00 for 30 or more years night shift work.
On average, one in seven women in the UK have worked night shifts at some point in their lives and one in 50 have worked nights for 20 or more years.
Professor Andrew Curran, chief scientific adviser for the Health and Safety Executive (HSE), which commissioned the study, said: “Breast cancer is the most common cancer in women so it was vital for us to fund work in this area to establish if there is a link to night work.
“In Great Britain, there are 2 million women, about one in six female workers, who are currently working in some types of shift work, and over half a million of them are working in shifts that involve night work.
“This study has shown that night shift work, including long-term shift work, has little or no effect on breast cancer incidence in women.
“However, there are a number of other known risks with shift work that employers must take into consideration when protecting their workers’ health and safety.”
Sarah Williams, Cancer Research UK’s health information manager, added: “Women can reduce their risk of breast cancer by keeping a healthy weight, drinking less alcohol and being active."