It’s long been known that working the night shift is associated with a high risk of disease, including heart disease, depression, insomnia, obesity, and, now research tells us, cancer.

Women who work night shift have an increased risk of contracting several types of cancer, including breast, skin, and gastrointestinal cancer, according to a study published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.

Xuelei Ma, PhD, oncologist, at State Key Laboratory of Biotherapy and Cancer Centre at West China Medical Center of Sichuan University, China has integrated a large number of previous studies, and 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,” she told The American Association for Cancer Research.

Previous research has looked at the association between working night shift and breast cancer, but Ma’s study looked at whether long-term night shift work in women was associated with risk for 11 types of cancer.

Ma and her colleagues used data from 61 articles – consisting of 114,628 cancer cases and 3,909,152 participants from North America, Europe, Australia, and Asia.

Long-term female night shift workers have 19% increased risk of cancer

The research showed that women who work night shift long term have a 19 per cent higher risk of contracting cancer.

Looking at specific cancers, the researchers found that women who worked long-term night shift had a 41 per cent increased risk of contracting skin cancer, 32 per cent increased risk of breast cancer, and 18 per cent increased risk of gastrointestinal cancer compared with women who did not perform long-term night shift work.

Curiously, the data showed that the increased risk of breast cancer was only found among female night shift workers in North America and Europe.

“We were surprised to see the association between night shift work and breast cancer risk only among women in North America and Europe,” said Ma.

“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,” she said.

The study also looked at female nurses alone, examining long-term night shift work for nurses and the risk of six types of cancer.

Nurses have highest risk of developing breast cancer of occupations studied

This study found nurses who worked long-term night shift had a 58 per cent higher risk of contracting breast cancer than nurses who didn’t work night shift.

Nurses who work night shift have a 35 per cent increased risk of gastrointestinal cancer, and 28 per cent increased risk of lung cancer, compared with nurses who did not work night shifts.

Of all the occupations studied, nurses had the highest risk of developing breast cancer if they worked the night shift.

Ma said that nurses might be more likely to undergo screening examinations, and therefore have a higher rate of cancer diagnosis.

“Another possible explanation for the increased cancer risk in this [nurse] population may relate to the job requirements of night shift nursing, such as more intensive shifts,” she said.

The researchers found that the risk of breast cancer increased by 3.3 per cent for every five years of night shift work.

“Our study indicates that night shift work serves as a risk factor for common cancers in women,” said Ma.

“Long-term night shift workers should have regular physical examinations and cancer screenings,” said Ma, who hopes the study will draw public attention to the issue.