In recent years, sleep research has expanded, with numerous animal studies examining how insufficient sleep contributes to diseases like diabetes, obesity, Alzheimer’s, and immune disorders and how these conditions affect sleep. Mice have often been used to test the effectiveness and side effects of new drugs, including sleep medications.
Rachel Rowe, senior author and assistant professor of integrative physiology, highlighted that while lifestyle factors and caregiving roles have long been thought to influence sleep differences between men and women, their research suggests that biological factors may play a more significant role than previously recognized.
According to a new animal study by CU Boulder researchers, females sleep less, wake up more often, and get less restorative sleep than males.
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 has focused primarily on males for decades.
First author Grant Mannino, who graduated with degrees in psychology and neuroscience, said, “Essentially, we found that the most commonly used mouse strain in biomedical research has sex-specific sleep behavior and that a failure to account for these sex differences properly can easily lead to flawed interpretations of data.”
In a noninvasive study, researchers used specialized cages with ultrasensitive movement sensors to monitor the sleep patterns of 267 C57BL/6J mice. They found that male mice slept about 670 minutes per 24-hour period, roughly an hour more than female mice.
The extra sleep was primarily non-rapid Eye Movement (NREM) sleep, which is vital for the body. Mice, nocturnal and polyphasic sleepers, nap briefly throughout the night before waking to survey their environment.
The study revealed that female mice had shorter, more fragmented sleep bouts. Similar sex differences in sleep patterns have been observed in other animals, such as fruit flies, rats, zebrafish, and birds, suggesting evolutionary significance.
Rachel Rowe suggested that biologically, female mice may be more sensitive to their environment, remaining alert when necessary due to their role in caring for the young. This sensitivity, she explained, could be an evolutionary adaptation, as women’s sleep patterns are influenced by factors like stress hormones (e.g., cortisol) and sex hormones.
For example, women often report poorer sleep during the phase of their menstrual cycle when estrogen and progesterone levels are lowest.
Some theories suggest that females may inherently need less sleep. Still, Rowe raised the question of whether women are creating unnecessary stress by comparing their sleep patterns to those of their partners and perceiving their sleep as poor when it might be normal.
The authors hope their findings will encourage further research into the biological differences between male and female sleep patterns. More importantly, they aim to inspire scientists to reconsider how they approach sleep research, particularly by accounting for these sex-based differences in their studies.
In 2016, the National Institutes of Health required researchers applying for funding to consider “sex as a biological variable” in animal studies. While progress has been made, sex bias remains, and this can have significant consequences.
The authors found that when testing a sleep treatment that worked best for females, the results were only accurate if the sample size included equal numbers of males and females. If females are underrepresented, treatments may appear ineffective for them, or side effects may go unnoticed.
Rowe pointed out that the lengthy pipeline from animal studies to clinical trials might be partly due to the insufficient consideration of sex differences. She and the authors urge researchers to include both sexes equally in studies, analyze data separately for each sex, and reassess past studies that have underrepresented females.
Rowe emphasized that the most surprising finding was not the difference in sleep patterns between male and female mice but that this was not thoroughly demonstrated until 2024.
Journal Reference:
- Mannino, G.S., Green, T.R.F., Murphy, S.M. et al. The importance of including both sexes in preclinical sleep studies and analyses. Sci Rep 14, 23622 (2024). DOI: 10.1038/s41598-024-70996-1