Working night shifts might increase the risk of irregular periods, hormone problems, and pregnancy complications, according to new research being presented at ENDO 2025, the Endocrine Society’s annual meeting in San Francisco.
The study, led by Dr. Alexandra Yaw, a postdoctoral fellow at Michigan State University, explored how changes in light exposure—similar to shift work—can affect the female reproductive system.
The research used a mouse model to mimic rotating light schedules, which reflects how real-life shift work can disrupt the body’s internal clock, or circadian rhythm.
To simulate this, the researchers changed the mice’s daily light-dark cycles by six hours every four days over several weeks. This disruption caused noticeable changes in the reproductive health of the female mice.
Interestingly, only half of the mice developed irregular menstrual-like cycles. These mice also showed signs of hormonal imbalance and poor ovarian health.
However, even the mice that continued to have regular cycles were not in the clear. The study found that the timing of both the ovaries and the uterus was affected in all mice exposed to the rotating light pattern.
To see if the disrupted light cycles would impact pregnancy, the researchers allowed the mice to mate.
All of the female mice, including those exposed to the “shift work” lighting, were able to become pregnant.
But there was a clear downside—those in the shift-work group had smaller litters and were more likely to experience problems during labor.
“This study helps explain the hidden reproductive risks associated with shift work,” said Dr. Yaw. “Some people’s bodies are more vulnerable to disruptions in light exposure, while others are more resilient.
This resilience may be linked to how well their brains and bodies can keep hormonal levels steady despite disruptions to their circadian rhythms.”
The findings suggest that even if a woman’s menstrual cycle appears normal, night shift work may still quietly impact her reproductive health. These results highlight the need for more research on how working irregular hours can affect pregnancy and fertility in the long run.
In the future, the researchers plan to study how shift-like light exposure affects the uterus during pregnancy, which could provide more insight into why it leads to difficult labor.
Ultimately, the goal of this research is to help women make informed decisions about their work schedules and to find ways to reduce the health risks linked to shift work.
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