Night shift work has long been linked to poor health. Now, a new study suggests that for women, such working patterns could increase the risk of cancer.

Researchers found that women who engaged in long-term night shifts were almost a fifth more likely to develop cancer when compared with women who did not work such shifts.

On looking at specific cancers, the researchers found that women who worked long-term night shifts were at even greater risk of skin cancer, breast cancer, and lung cancer.

Study co-author Xuelei Ma, PhD, of the West China Medical Center at Sichuan University in China, and colleagues recently reported their findings in the journal Cancer Epidemiology, Biomarkers & Prevention.

According to the Centers for Disease Control and Prevention (CDC), nearly 15 million people in the United States have irregular working patterns, including night shifts.

Previous research has shown that night shift work can severely disrupt our circadian rhythm. This is the internal clock that regulates the body's physiological processes over a 24-hour cycle, and it primarily responds to light and dark in the environment.

Shift work has been linked to a wealth of poor health outcomes, including obesity, heart disease, and some types of cancer.

In the new study, Ma and colleagues sought to learn more about the link between long-term night shift work and cancer risk.

Overall cancer risk increased by 19%

The researchers came to their findings by conducting a meta-analysis of 61 studies. These included a total of 3,909,152 participants and more than 114,000 cancer cases. Subjects were from Asia, Australia, Europe, and North America.

The team used these data to assess how long-term night shift work was associated with the risk of developing 11 cancer types.

In a separate analysis, the researchers looked at whether long-term night shift work among female nurses was linked to the risk of six cancer types.

Overall, the new study revealed that women who worked long-term night shifts were at 19% greater risk of cancer, compared with women who did not work long-term night shifts.

Additionally, the researchers found that women who engaged in long-term night shift work had a 41% greater risk of skin cancer, a 32% greater risk of breast cancer, and an 18% increased risk of gastrointestinal cancer.

In a dose-response analysis, it was also found that each additional 5 years of night shift work was associated with 3.3% increase in breast cancer risk, the researchers report.

Further investigation, however, revealed that the link between night shift work and greater breast cancer risk was only applicable for women who lived in North America or Europe.

"It is possible that women in these locations have higher sex hormone levels," Ma suggests, "which have been positively associated with hormone-related cancers such as breast cancer."

'A risk factor for common cancers'

From the analysis of female nurses alone, the researchers found that long-term night shift work was linked to a 58% greater risk of breast cancer, a 35% increased risk of gastrointestinal cancer, and a 28% greater lung cancer risk.

Discussing the possible reasons for the increased cancer risk among female nurses, Ma speculates that these women may have been more likely to undergo cancer screening due to their profession.

"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," adds Ma.

Overall, the scientists say that their findings indicate that more needs to be done to protect the health of female shift workers, particularly those who engage in such shifts for many years.

"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."

Xuelei Ma, PhD

Ma adds that he hopes that this research will encourage larger cohort studies, in order to confirm the link between long-term night shift work and cancer among women.