The science of sleep is woefully incomplete, not least because research on the topic has long ignored half of the population. For decades, sleep studies mostly enrolled men. Now, as sleep researchers are making a more concerted effort to study women, they are uncovering important differences between the sexes.

Hormones are a major factor. Estrogen, progesterone and testosterone can influence the chemical systems in the brain that regulate sleep and arousal. Moreover, recent studies indicate that during times of hormonal change—such as puberty, pregnancy and menopause—women are at an increased risk for sleep disorders such as obstructive sleep apnea, restless legs syndrome and insomnia. Women also tend to report that they have more trouble sleeping before and during their menstrual periods.

And when women do sleep poorly, they may have a harder time focusing than sleep-deprived men do. In one recent study, researchers shifted the sleep-wake cycles of 16 men and 18 women for 10 days. Volunteers were put on a 28-hour daily cycle involving nearly 19 hours of awake time followed by a little more than nine hours of sleep. During the sleep-shifted period, the women in the group performed much less accurately than the men on cognitive tests. The findings, published in April of this year in the Proceedings of the National Academy of Sciences USA, may help explain why women are more likely than men to get injured working graveyard shifts. In addition, a study conducted in 2015 in teenagers reported that weekday sleep deprivation affects cognitive ability more in girls than in boys.

And yet despite the fact that sleep issues may affect women disproportionately, they may also be underrecognized in women. Statistics suggest that men are more than twice as likely to be diagnosed in youth or middle age with obstructive sleep apnea—a disorder characterized by periodic stops and starts in breathing during sleep. This may be in part because the diagnostic criteria are skewed toward men. “We discovered the disease in a specific sex, so of course it's going to be more typically catered to the features seen in that sex,” explains Christine Won, director of the Women's Sleep Health Program at the Yale School of Medicine. Men are often flagged for possible apnea after reporting to their doctor that they snore or gasp awake during the night, but a study earlier this year found that women with sleep apnea have different symptoms, such as daytime sleepiness, which their doctors may not recognize as apnea-related.

Even when women do undergo sleep testing, they still may not be properly diagnosed. Apnea tends to cluster during the rapid eye movement (REM) stage of sleep in women, whereas male apnea is not as stage-specific, Won explains. Because apnea is diagnosed by calculating an average index of breathing issues during a total night's sleep—an approach that was, again, built on studies involving men—the severity of women's REM-focused apnea often gets diluted, which is especially worrying considering the results of a 2015 study that found that women with sleep apnea are at an increased risk for heart failure and death as compared with men. “We have overrelied on screening instruments, devices and pharmacological agents that were designed for men that may not be applicable to women,” explains Monica Mallampalli, vice president of Scientific Affairs at the nonprofit Society for Women's Health Research in Washington, D.C. The balance is finally shifting, but it may take decades for scientists to uncover all the sex differences related to sleep and update the diagnostic criteria, statistics and treatments to reflect them.

Who Gets the Most Sleep?

Researchers at the University of Michigan used a free smartphone app to collect bedtime and wake-time data from 8,000 people in 128 countries. No big surprise: most people fail to get the solid eight hours that experts often recommend. Folks in Singapore average the least sleep, and the Dutch get the most. Other findings:

  • Women average 30 minutes more snooze time than men.
  • Middle-aged men get the least shut-eye.
  • Bedtime, not wake time, makes the biggest difference in how much sleep you ultimately get.