New research suggests that sleep disorders should be approached differently based on biological sex, as men and women exhibit distinct sleep patterns. A recent literature review, conducted by researchers from Harvard University, Stanford University, and the University of Southampton in the U.K. and published in April in the journal Sleep Medicine Reviews, found that men are more prone to obstructive sleep apnea, while women are more likely to suffer from insomnia and report poorer sleep quality.

Coauthor Renske Lok, PhD, a postdoctoral fellow at the Stanford Center for Sleep and Circadian Sciences, describes this research as focusing on both precision medicine and the differences in sleep between genders.

“We’re moving towards personalized medicine,” she tells Fortune. “[Medicine] should be more individualized.”

Understanding the impact of biological sex on various sleep disorders is crucial for tailoring treatments effectively. However, the historical underrepresentation of women in biomedical and behavioral research has posed a significant challenge. It wasn’t until 2016 that the National Institutes of Health mandated the inclusion of sex as a biological variable in studies.

Lok emphasizes the importance of improving the inclusion of women in research designs, noting that historically, women have been underrepresented. She highlights the misconception that findings from studies on men would apply universally to women, which is increasingly being disproven.

“The key finding is that we must improve the inclusion of women in our research designs,” Lok says. “Women have been historically underrepresented, partly due to the assumption that results from studies on men would automatically apply to women. We are discovering more and more that this is not the case.”

Sex and circadian rhythm

Circadian rhythms refer to the mental, physical, and behavioral changes that occur in a 24-hour period. Nearly all organs and tissues have their own rhythms, collectively forming a master biological clock highly sensitive to light and dark cycles.

During the night, the brain increases production of melatonin, the sleep hormone, inducing feelings of tiredness. According to research reviewed by Lok and her colleagues, women typically secrete melatonin earlier in the evening than men. This finding corresponds with other studies indicating that men tend to have later chronotypes, meaning they go to bed and wake up later than women. Consequently, men often experience more severe social jetlag, where their biological clock conflicts with societal norms, such as traditional work hours.

Additionally, studies show that women have an earlier peak in core body temperature before sleep compared to men. Furthermore, women have slightly shorter circadian periods than men, approximately six minutes less: 24.09 hours versus 24.19 hours.

Although this discrepancy may seem minor, it holds significance. The misalignment between the central body clock and the sleep/wake cycle is approximately five times greater in women than in men. Lok emphasizes the potential health implications of such disruptions, including sleep disorders, mood disorders, and impaired cognitive function.

To realign circadian rhythms, cognitive behavioral therapy can be effective, especially for individuals with discordant biological and social clocks. Alaina Tiani, PhD, a clinical psychologist at the Cleveland Clinic Sleep Disorders Center, suggests strategies such as taking melatonin supplements earlier in the evening and incorporating bright-light exposure in the morning, particularly beneficial for night owls needing to adjust their waking times.

Work-life stress may influence women’s insomnia

You’ve likely encountered episodes of acute insomnia during stressful periods in your life, characterized by difficulties falling asleep, staying asleep, or achieving restful sleep. These episodes may have persisted for a few days or extended up to several weeks. Chronic insomnia, on the other hand, is defined as experiencing these sleep disturbances at least three times a week for over three months, according to the National Heart, Lung, and Blood Institute. Chronic insomnia cannot be attributed to other underlying health issues.

Previous research has indicated that insomnia is approximately 1.5 times more prevalent in women. Lok and her colleagues hypothesized that this gender difference may be linked to specific risk factors more common in women, such as anxiety and depression.

Dr. Eric Sklar, a neurologist and medical director of the Inova Sleep Disorders Program in northern Virginia, highlights the strong association between insomnia and underlying psychiatric disorders. He suggests that societal stressors affecting men and women may differ, with women often shouldering the dual responsibilities of family caregiving and professional advancement, alongside other life stresses. Sklar notes the importance of evening relaxation time for maintaining healthy circadian rhythms, which may be more challenging for women due to competing demands and the phenomenon of “revenge bedtime procrastination,” where screen time interferes with sleep patterns.

According to the review, women tend to exhibit better sleep efficiency, spending a higher percentage of time in bed asleep compared to men. They also enter the rapid eye movement (REM) phase of sleep earlier and spend approximately eight minutes longer in non-REM sleep. However, despite these objective measures, women self-report poorer sleep quality than men.

Alaina Tiani, a clinical psychologist at the Cleveland Clinic Sleep Disorders Center, notes that many postpartum patients and women with young children report diminished sleep quality, often feeling as though their minds are partially alert during the night, anticipating their children’s needs. Similarly, caregivers in other roles have reported similar experiences of being attuned to potential nighttime demands.

Why do men and women sleep differently?

Women have a comparative advantage when it comes to one prevalent sleep disorder: obstructive sleep apnea, a condition where the upper airway is recurrently obstructed during sleep. While the disorder is nearly three times more common in men, it’s associated with an elevated risk of heart failure specifically in women, as highlighted in the review.

Dr. Sklar emphasizes the well-established higher risk for men, citing biological sex as a factor in assessing sleep apnea risk. He explains that men’s larger neck sizes contribute to this heightened risk.

Lok’s review outlines several sleep disparities between genders:

• Women are 1.5–4 times more likely to experience a sleep-related eating disorder.
• Restless legs syndrome is 25–50% more common in women.
• Women report more variability in sleep quality.
• Men have less consistent rest-activity patterns.
• Men tend to overeat more in response to sleep loss.
• Male night-shift workers face a greater risk of developing Type 2 diabetes.

However, one aspect that remains inconsistent across the numerous studies analyzed by Lok and her team is women’s menstrual phases. Menstruation triggers various sleep-affecting changes, such as elevated body temperature during the luteal phase. Additionally, some studies fail to account for subjects’ use of oral contraceptives, which could skew results.

Lok acknowledges the complexity of accounting for menstrual phases, particularly in studies involving hormonal contraceptives. Ensuring consistency in menstrual phase representation among participants is crucial to avoid confounding factors stemming from hormonal fluctuations.

Despite challenges in studying biological sex differences due to limited evidence, Lok expresses optimism about future research endeavors. While some disparities remain unexplored, she finds encouragement in identifying areas where knowledge gaps persist.