You know what Ben Franklin said: Early to bed, early to rise makes a woman rested, safe, and married.

A new study from the University of Chicago shows that women who stay up late tend to have similar risk-taking tendencies as men, and that night-owls of both genders were less likely to be in long-term relationships.

Researchers found that men generally have higher levels of cortisol and testosterone than women, but that night-owl women have just as much cortisol as men. High levels of cortisol are usually associated with high energy, arousability, stress and even cognitive function, and some research has shown that successful people usually have higher cortisol levels. The researchers found that high cortisol levels may explain why night-owls take more risks.

In other words, women who stay up late tend to get laid more often, but women who go to bed early and get up early might be more likely to be in stable relationships.

Apparently the tendency to stay up late may have been an evolutionary trait that enabled our caveman ancestors to get frisky after the kids went to bed. “From an evolutionary perspective, it has been suggested that the night-owl trait may have evolved to facilitate short-term mating, that is, sexual interactions that occur outside of committed, monogamous relationships,” lead researcher Dario Maestripieri told UChicago News. “Being active in the evening hours increased the opportunities to engage in social and mating activities, when adults were less burdened by work or child-rearing.”

By the way, men who are night-owls have twice as much sex as men who are early birds. But since sleep loss might cause brain damage, you might have to choose between sex partners and brain cells.

[Journal of Evolutionary Psychology]

Write to Charlotte Alter at charlotte.alter@time.com.