Biological factors may play a larger role in sleep differences than previously understood, according to new findings that could reshape the landscape of biomedical research. The study highlights a significant oversight in animal research—failure to account for sex differences in sleep behavior—which may lead to flawed data interpretations.
“Essentially, we found that the most commonly used mouse strain in biomedical research has sex-specific sleep behavior, and a failure to properly account for these differences can easily skew results,” said Grant Mannino, the study’s first author and a psychology and neuroscience graduate.
The researchers studied 267 “C57BL/6J” mice, a widely used strain in biomedical studies, using non-invasive cages equipped with ultrasensitive movement sensors. Their findings revealed that male mice slept approximately 670 minutes per 24-hour period—about an hour more than females. This extra sleep occurred during non-Rapid Eye Movement (NREM) sleep, the restorative phase crucial for bodily repair.
Female mice, on the other hand, exhibited shorter and more fragmented sleep cycles. This pattern aligns with findings in other animals, including fruit flies, rats, zebrafish, and birds. Evolutionary factors may contribute to this discrepancy.
“From a biological standpoint, it could be that females are designed to be more sensitive to their environment and wake up easily to care for their young,” said researcher Rowe. “If females slept as deeply as males, we wouldn’t move forward as a species.”
Hormones like cortisol and sex hormones such as estrogen and progesterone also appear to influence sleep patterns. Women, for instance, often report poorer sleep during phases of their menstrual cycle when these hormones are at their lowest.
The findings underscore a critical need for more inclusive research. Many prior studies—particularly those involving mice—relied heavily on male subjects, potentially skewing results and impacting drug development.
In 2016, the National Institutes of Health (NIH) mandated that scientists account for “sex as a biological variable” when applying for funding for animal studies. While progress has been made, sex bias in research persists, with tangible consequences.
The study simulated a sleep treatment effective primarily in females and found that its efficacy was only evident when equal numbers of males and females were included in the sample. Without balanced representation, treatments that benefit females might appear ineffective, and side effects impacting them more severely could be overlooked.
The authors urge scientists to prioritize sex parity in research and re-analyze previous studies that excluded females. “The pipeline from bench to bedside often spans decades. If sex differences aren’t adequately considered, could this delay progress?” asked Rowe.
These findings could serve as a wake-up call for the scientific community. By addressing sex bias, researchers can ensure that medical advancements benefit everyone equally.
“The most surprising finding isn’t that male and female mice sleep differently,” Rowe added. “It’s that no one has thoroughly documented this until now. We should have known this long before 2024.”
By Impact Lab
