Asthma is a chronic respiratory disease that often causes sudden and intense shortness of breath. The disease impacts around 339 million people worldwide, and in the U.S. nine people on average die from asthma every day.

Researchers in the U.K. have found that women who work night shifts are more likely to have moderate or severe asthma than women who work during the day. The study, published Monday in the journal ERJ Open Research, does not reveal any such tendency in men working nightshifts, and could hold important implications for public health guidance.

The team had previously shown that permanent night shift workers in general are more likely to suffer from moderate-severe asthma compared to day workers. Scientists also know that asthma is more prevalent and severe in adult women than adult men. “Since increasing numbers of females are becoming shift workers it is important to determine if shift work-associated asthma risk is higher in females,” the researchers wrote in the study. Shift work refers to scheduled working periods that fall outside of traditional daytime hours. “Our main aim was to investigate sex differences in the association between shift work and asthma.”

The researchers did so by analyzing data belonging to over 274,541 working people from the U.K. Biobank, a biomedical database open to health researchers. From this initial pool, they found that 5.3% had asthma, and 1.9% of those had moderate or severe asthma. The team then looked into whether the people in these categories worked during the day, during nightshifts, or a mix of both.

The researchers found that female shift workers in general are more likely to suffer from asthma, and women who only work nightshifts have 50% higher likelihood of developing moderate or severe asthma than female daytime workers. Furthermore, the risk increases “with both number of monthly night shifts and longer lifetime duration of night shift work,” they explained. Interestingly, the data did not reveal any differences in the prevalence of asthma in men based on their work schedule.

“This type of research cannot explain why shift work and asthma are linked,” Robert Maidstone, a co-author of the study and a researcher at the University of Manchester’s School of Biological Sciences, said in a university statement. “However, it could be because shift work disrupts the body clock, including the levels of male and female sex hormones. High testosterone has previously been shown to be protective against asthma, and so lower testosterone in women could play a role. Alternatively, men and women work different types of shift jobs, and this could be a factor.”

Among women not taking hormone replacement therapy (HRT, a treatment for menopausal symptoms), postmenopausal women working only night shifts were almost twice as likely to have moderate-severe asthma than female day workers. This suggests that HRT might protect nightshift workers from asthma, though the researchers admit that more studies are needed to confirm this.

“This research suggests that working nightshifts could be a risk factor for asthma in women, but not in men. The majority of workers will not have an easy option of switching their shift pattern, so we need further research to verify and understand this link and find out what could be done to reduce the risk for women who work shifts,” explained Florence Schleich from the European Respiratory Society, who was not involved in the study.

Moving forward, the researchers aim to investigate how sex hormones might influence asthma prevalence among shift workers. Perhaps this line of research will also shed light on what drives the respiratory disease, given that the exact cause remains a mystery.