Working night shifts can increase the risk of cancer for women by a fifth, according to new research.
Scientists found it could increase the risk of skin cancer by 41 per cent, breast cancer by 32 per cent, and stomach cancer by 18 per cent.
Nurses had the highest increase in risk – 58 per cent – of developing breast cancer if they worked the night shift, the analysis suggested.
Researchers hoped the findings could lead to measures to protect female night shift workers in an increasingly 24 hour society.
Earlier studies looked at the link between female night shifters and the risk of breast cancer – the most diagnosed worldwide for women – but the conclusions have been inconsistent.
The new research looked at whether long-term night shift work in women was associated with risk for nearly a dozen types of cancer.
Shift work stats
The team carried out a meta-analysis using data from 61 articles comprising 114,628 cancer cases and 3,909,152 participants from North America, Europe, Australia, and Asia.
These studies were analysed for an association between long-term night shift work and risk of 11 types of cancer.
A further analysis was conducted, which looked specifically at long-term night shift work and risk of six types of cancer among female nurses.
After dividing the participants by location, the analysis showed an increased risk of breast cancer was only found among female night shift workers in North America and Europe.
Assistant professor Xuelei Ma, from West China Medical Center of Sichuan University, in Chengdu, China, said: “We were surprised to see the association between night shift work and breast cancer risk only among women in North America and Europe.
“It is possible that women in these locations have higher sex hormone levels, which have been positively associated with hormone-related cancers such as breast cancer.”
Female nurses who worked night shifts had a 58 per cent increased risk of breast cancer, 35 per cent of gastrointestinal, and 28 per cent of lung cancer, compared with those that did not work night shifts.
Of all the occupations analysed, nurses had the highest risk of developing breast cancer if they worked the night shift.
Prof Ma said: “Nurses that worked the night shift were of a medical background and may have been more likely to undergo screening examinations.
“Another possible explanation for the increased cancer risk in this population may relate to the job requirements of night shift nursing, such as more intensive shifts.”
The researchers also performed a dose-response meta-analysis among breast cancer studies that involved three or more levels of exposure.
They found that the risk of breast cancer increased by 3.3 per cent for every five years of night shift work.
Prof Ma added: “By systematically integrating a multitude of previous data, we found that night shift work was positively associated with several common cancers in women.
“The results of this research suggest the need for health protection programs for long-term female night shift workers.
“Our study indicates that night shift work serves as a risk factor for common cancers in women.
“These results might help establish and implement effective measures to protect female night shifters. Long-term night shift workers should have regular physical examinations and cancer screenings.
“Given the expanding prevalence of shift work worldwide and the heavy public burden of cancers, we initiated this study to draw public attention to this issue so that more large cohort studies will be conducted to confirm these associations.”
However, researchers said there was a lack of consistency between studies regarding the definition of “long-term” night shift work.
These could include “working during the night”, and “working at least three nights per month”.
The articles consisted of 26 cohort studies, 24 case-control studies, and 11 nested case-control studies.
The study was published in the journal Cancer Epidemiology, Biomarkers & Prevention.