That women suffer from insomnia more than men is a pretty well-known fact, and as women age, it seems to become even more pronounced. But the cause of the gender divide, like many other aspects of health and mental health, hasn’t been totally clear. A new study, however, looks into how the sleep-wake cycle works in women and men under normal conditions–and understanding the difference in healthy individuals actually reveals a lot about why women have more trouble sleeping and why they wake up earlier than men.

The team from McGill University had 15 men and 11 women come and spend 36 hours in the lab. They subjected the participants to a “cat nap” paradigm, where they’d raise and lower the lights, so that the participants slept for an hour and rose for an hour. They measured levels of melatonin, the central sleep hormone and other, subjective variables, like wakefulness and sleepiness. They made sure to study women during two phases of their reproductive cycles, since sex hormones can affect sleep quality.

They found that women’s sleep cycles ran at an earlier phase than men’s–that is, their sleepiness tended to set in earlier, and their nighttime alertness was lower than men’s. As the authors write in their paper, published in Proceedings of the National Academy of Sciences, this essentially means that women are beginning their sleep at a later time than men, relative to how their bodies are set. And this mismatch may explain why women have a harder time falling asleep and staying asleep than men.

“For a similar sleep schedule, we find that women’s body clock causes them to fall asleep and wake up earlier than men,” said study author Diane B. Boivin in a statement. “The reason is simple: their body clock is shifted to a more easterly time zone. This observed difference between the sexes is essential for understanding why women are more prone to disturbed sleep than men.”

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