Women who work night shifts may have an increased risk of irregular periods and hormonal imbalances, according to a study being presented Monday at ENDO 2025, the Endocrine Society’s annual meeting in San Francisco, Calif.

“Shiftwork-like light exposure disrupts the body’s internal timing, causing a split response where some females have disrupted reproductive cycles and hormones while others do not, but both groups face increased risk of ovarian disruption and pregnancy complications, including difficult labor, in response to shift work-like light exposure,” said Alexandra Yaw, Ph.D., a postdoctoral fellow in the Department of Animal Science at Michigan State University in East Lansing, Mich.

Yaw and colleagues used a mouse model of rotating light shifts that mimics changing light patterns to understand how shift work affects the reproductive system. Specifically, the researchers started and delayed the 12-hour light to 12-hour dark cycle for 6 hours every 4 days for 5 to 9 weeks.

Half of the female mice exposed to the shiftwork lights developed irregular cycles, while the others continued cycling normally. Those with irregular cycles also had hormonal imbalances and signs of poor ovarian health.

However, the shiftwork lighting disrupted the timing of the ovaries and uterus, even in mice with normal cycles.

To understand whether the rotating light affected pregnancy, they mated the mice. They found all the mice, even the “shift workers,” were able to get pregnant, but all mice exposed to the shift work lighting had smaller litters and a much higher chance of having complications during labor.

“This study helps explain the hidden reproductive risks associated with shift work,” Yaw said. “While everyone reacts differently, some are more vulnerable than others. The resilience among some may depend on how their brain and body maintain hormonal balances despite disruptions to their circadian rhythm.”

In the long term, the researchers hope this work helps women protect their fertility and pregnancy outcomes and empowers them to make informed decisions about their health and work schedules.

Future studies looking at how the pregnant uterus works in the model will be important for figuring out how rotating light shifts cause difficult labor, the researchers said.