Men and women sleep differently, and we finally know why

Have you ever laid awake in bed, glancing at your peacefully sleeping partner, and wondered why sleep seems to elude you despite your best efforts? It turns out this is a common experience for women.

A recent study conducted by researchers at CU Boulder indicates that women tend to sleep less, wake up more frequently, and enjoy less restorative sleep than men.

Bridging gaps in sleep research

Interestingly, these latest insights shed new light on what might be responsible for variations between men and women’s sleep patterns. The implications are quite broad and potentially game-changing for biomedical research, which has been primarily centric to the male sleep pattern for many years.

“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 revolution and its gender gap

In recent years, sleep research has truly experienced a boom. Thousands of animal-based studies are now exploring how sleep deprivation can increase the risk of a host of health conditions – from diabetes and obesity to Alzheimer’s and immune disorders.

Yet, an interesting aspect that might have gone unnoticed for a while is the potential skewness of these results due to a significant underrepresentation of females in these studies.

Study first author Grant Mannino graduated with degrees in psychology and neuroscience and was named outstanding undergraduate of the College of Arts and Sciences.

“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 Mannino.

Studying sleep patterns in mice

In their non-invasive study, the research team utilized specially-created movement-sensitive cages to understand the sleep patterns of 267 “C57BL/6J” mice. The results were fascinating.

Male mice slept approximately 670 minutes within a 24-hour period, around an hour longer than their female counterparts. This extra snoozing can be attributed to non-Rapid Eye Movement (NREM) sleep, a crucial restorative sleep phase when the body repairs itself.

Notably, mice are “polyphasic sleepers,” taking short naps, waking for brief periods to check their surroundings, and then returning to their slumber. The researchers discovered that female mice have even smaller sleep durations with their sleep being more fragmented.

Sleep differences in men and women

This sex-biased sleep occurrence isn’t unique to mice. It has been observed in various species from fruit flies and rats to zebrafish and birds. This pattern might seem bewildering, but it makes perfect evolutionary 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?”

Why do women sleep less?

But what’s the culprit behind these differences?

It seems that stress hormones, such as cortisol, and sex hormones may be major influencers. Women often report worse sleep during their menstrual cycles when estrogen and progesterone levels are lowest.

There are hypotheses suggesting women may inherently require less sleep. However, Rowe poses an intriguing question: “Are we inducing undue stress on ourselves thinking we sleep inadequately because we compare ourselves to our male partners?”

Calls for equitable research

The authors from CU Boulder hope that their findings will prompt a greater focus on understanding the biological variances in sleep patterns between the sexes. More importantly, they also hope it leads to a reevaluation of research methodology.

In 2016, the National Institutes of Health required scientists to consider “sex as a biological variable” when applying for funding for animal studies. Although progress has been made, recent research indicates that a bias towards males still exists.

This bias can have serious implications. For instance, if female representation is low in clinical trials, drugs that are more effective for women may appear ineffective, or severe side effects might be overlooked.

Finally, the research team urges fellow scientists to consider equal representation of both sexes in their studies, analyze data separately for men and women, and reevaluate past research on sleep where female representation was insufficient.

“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.”

The study is published in the journal Scientific Reports.

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