Night shift work does not increase the risk of breast cancer, finds a UK study covering analysis of 102,869 women over 10 years.
In 2007, the International Agency for Research on Cancer (IARC) concluded that shift work disrupting the body’s sleep-wake cycle was “probably carcinogenic”. However, the agency is due to review its findings this summer.
Breast Cancer Now claims the latest research, which it funded, is the most comprehensive to date. Michael Jones, a co-author of the study and staff scientist in genetics and epidemiology at the Institute of Cancer Research, in London, said: “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.
“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.”
Breast cancer is the UK’s most common cancer, with about 55,000 women and 350 men being diagnosed each year. Previous research conclusions have differed on the impact of shift work.
In 2009, in response to the 2007 IARC research, Denmark began compensating dozens of women with the disease who said their illness had been brought on by working night shifts.
That study had been challenged due to the older average age of participants and because it had limited details on the nature of women’s shift work.
The median age of participants at the age of recruitment in the latest study was 45 years, and 17.5% of participants reported being night shift workers regularly working between 10pm and 7am within the previous 10 years. The shift data was followed up six years later.
The researchers observed that 2,059 out of 102,869 women went on to develop invasive breast cancer. Taking into account confounding risk factors, the researchers found no overall association with night shift work.
They also found no significant difference in risk relating to the type of night shift work, the age at which the work was started, or whether it began before or after a first pregnancy.
The only statistically significant trend was found specifically with average night hours worked per week, but researchers said it was not supported by previous evidence or any proposed biological explanation.
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.”