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Females sleep less, wake up more often and get less restorative sleep than males, according to a new animal study by CU Boulder researchers.

The findings, published in the journal Scientific Reports, shed new light on what may underlie sleep differences in men and women and could have broad implications for biomedical research, which for decades has focused primarily on males.

“In humans, men and women exhibit distinct sleep patterns, often attributed to lifestyle factors and caregiving roles,” said senior author Rachel Rowe, assistant professor of integrative physiology. “Our results suggest that biological factors may play a more substantial role in driving these sleep differences than previously recognized.”

Sleep research has exploded in recent years, with thousands of animal studies exploring how insufficient sleep impacts risk of diseases like diabetes, obesity, Alzheimer’s and immune disorders—and how such diseases impact sleep. Meanwhile, mice have often been the first to be tested to see whether new drugs, including medications for sleep, work and what the side effects are.

But many of those results may have been skewed due to a lack of female representation, the study suggests.

“Essentially, we found that the most commonly used mouse strain in biomedical research has sex-specific sleep behavior and that a failure to properly account for these sex differences can easily lead to flawed interpretations of data,” said first author Grant Mannino, who graduated with degrees in psychology and neuroscience and was named outstanding undergraduate of the College of Arts and Sciences in May.

How mice sleep

For the non-invasive study, the authors used specialized cages lined with ultrasensitive movement sensors to assess the sleep patterns of 267 “C57BL/6J” mice.

Males slept about 670 minutes total per 24-hour period, about an hour more than female mice. That extra sleep was non-Rapid Eye Movement (NREM) sleep—the restorative sleep when the body works to repair itself.

Mice are nocturnal and are “polyphasic sleepers”—napping for a few minutes before arousing briefly to survey their environment and then resuming their slumber. Females, the study found, have even shorter bouts of sleep—essentially, their sleep is more fragmented.

Similar sex differences have been seen in other animals, including fruit flies, rats, zebrafish and birds. Evolutionarily, it makes sense.

“From a biological standpoint, it could be that females are designed to be more sensitive to their environment and be aroused when they need to be because they are typically the one who is caring for the young,” Rowe said. “If we slept as hard as males sleep, we would not move forward as a species, right?”

Rachel Rowe

Stress hormones like cortisol (which promotes wakefulness) and sex hormones likely play a role. For instance, women tend to report worse sleep during the time in their menstrual cycle when estrogen and progesterone are at their lowest.

Some have hypothesized that females inherently require less sleep.

“For me, the question is: Are we creating too much stress for ourselves because we don’t sleep as much as our husband or partner and think our sleep is poor when actually that is a normal sleep profile for ourselves?” said Rowe.

The authors hope their findings inspire more research into underlying biological differences. More importantly, they hope the study prompts scientists to re-evaluate how they do research.

Progress made but more work to be done

In 2016, the National Institutes of Health began requiring scientists applying for funding for animal studies to consider “sex as a biological variable.” Progress has been made, but research has shown that sex bias still exists. And it can have real consequences, the authors found.

When they simulated a sleep treatment that worked best in females, they found that it was accurately reflected only if the sample size was made up evenly of males and females.

Bottom line: If females are underrepresented, drugs that work best for them may seem ineffective, or side effects that hit hardest may go unnoticed.

“The pipeline from bench to bedside is decades-long and often things that work in animals fail when they get to clinical trials. Is it taking so long because sex isn’t being considered enough?” said Rowe.

The authors encourage researchers to include both sexes equally when possible, analyze data for males and females separately, and re-evaluate past studies that underrepresented females.

“The most surprising finding here isn’t that male and female mice sleep differently. It’s that no one has thoroughly shown this until now,” said Rowe. “We should have known this long before 2024.”