Why you should care
Because either you are a woman or you emerged screaming from one.
Sex ed is good for all kinds of things: naps, jokes and possibly actual education about birds versus bees. Some of us may have learned some science, like the fact that testosterone and estrogen determine whether we grow beards or breasts. But it turns out sex hormones affect more than how we look. In recent years, researchers have awakened to the possibility that they might even influence how we sleep.
A recent study led by Diane Boivin of McGill University suggests that differences in male and female biological clocks — which respond to sex hormones — might explain why. Boivin’s team found that women’s biological clocks ran about two hours ahead of men’s. In fact:
Insomnia may strike women more often than men.
In the study, women woke up and fell asleep earlier and were more likely to wake up late at night. Which isn’t to say that men don’t have sleep problems. Men, for instance, have a higher risk of sleep apnea than women. But “for whatever reason, women are choosing to sleep at times that predispose them to sleep-maintenance issues, and men aren’t,” says Jeanne Duffy of Harvard Medical School, who wasn’t involved in the study.
Few studies have examined the biological clock in women versus men. Those that have looked at these differences didn’t control for hormonal fluctuations due to the menstrual cycle or oral contraceptives like the pill, which earlier research had found affects core body temperature and other mechanisms that regulate the biological clock. To that end, she and her colleagues recruited 11 women who were in the same phase of their menstrual cycles (confirmed by measuring progesterone levels in their blood) and who weren’t on the pill. They also recruited 15 men.
The participants stayed in rooms sans windows, clocks or anything else that could clue them in to the time of day. They then slept for one or two nights so researchers could measure their baseline sleep levels to ensure they all had similar bedtimes and didn’t have sleep disorders. For the next three days, the participants switched between 60 minutes of awake time and 60 minutes of nap time. To track their biological clocks, the researchers measured their core body temperatures, levels of melatonin (the “drowsy hormone”), as well as various aspects of their sleep patterns, such as how long it took them to doze off and how deeply they slept. Participants indicated their alertness along a line spanning from extremely sleepy on the left to extremely alert on the right.
The end of the night represents “a peak period of vulnerability for sleep in women.”
On average, women’s biological clocks were advanced by about one hour compared to those of men. Their alertness also peaked one hour earlier in their biological day, and they felt more tired at night. In other words, they were hitting the hay at a later biological-clock time, suggesting that the end of the night — as one cycle of the biological clock ends and another begins — represents “a peak period of vulnerability for sleep in women,” Boivin says, which could help explain why they often awaken around that time and feel groggy even after a night of sleep.
To be sure, the researchers used a small sample size. Also, most of the women slept for one night before the 60-minute sleep-wake cycles, while most of the men slept for two nights. “That could have potentially influenced some of the sleep measures,” Duffy says, a limitation the study authors acknowledged in the paper. “Just sleeping in the lab for the first time, a lot of people don’t sleep well.” As a result, participants who slept two nights beforehand — mostly men — might have shown better quality sleep on the night before the 60-minute cycles and therefore felt less sleepy going into those cycles. Nonetheless, the study adds to other studies showing that regulation of sleep by the circadian system is different in women than in men, Duffy says.
The study doesn’t explain what causes these differences, but Boivin thinks the sex hormones estrogen, progesterone and testosterone play a role. The brain region primarily responsible for regulating the biological clock, the suprachiasmatic nucleus, contains receptors for these hormones. Studies have also shown that women are more likely to suffer from insomnia as they near menopause — when they experience a surge of estrogen. Why women evolved to wake up and sleep earlier is less clear. Some scientists propose that infants’ tendency to wake up early selected for mothers with similar sleep patterns who could ensure their survival.
For now, the findings suggest women might have a harder time working night shifts, which Boivin hopes to test in future studies. They also hint at possible interventions for sleep disturbances tailored to sex, like timed light exposure, Duffy says. In other words, ladies, it’s not surprising to find yourself wide awake in the wee hours (and, if you’re straight and coupled, shooting death glares at your partner, snoozing like a baby beside you). Sleep when you’re tired, even if it seems early.
You’re not a grandma — you’re just listening to your biological clock.