There's an old stereotype, based at least partly in truth, that men can tend to get a little bit sleepy after they orgasm during sex.
There are a number of reasons why this happens, and a few thoughts on what the evolutionary advantage of being a bit tired after having sex could be. First, let's deal with the biological aspect.
When you orgasm, you release a number of hormones, including oxytocin, prolactin, gamma-aminobutyric acid, and other endorphins. These are all things that will contribute to feelings of sleepiness, with oxytocin playing a key role.
“The hormone, which is released during orgasm, is associated with reducing stress, reducing defensiveness and fostering immobility without fear. That's the ideal situation for going to sleep,” Dr Sue Carter, of Indiana University's Kinsey Institute for Research in Sex, Gender and Reproduction, explained to HuffPost.
“To be able to lie still at night and not having your brain racing and thinking about the things that are bothering you, or that you have to do the next day, is ideal for quality rest.”
Prolactin is also thought to play a large role in sleepiness, as well as in decreasing arousal following sex. People with higher levels of prolactin have been found to have increased daytime sleepiness, suggesting a link between the hormone and feelings of drowsiness.
Interestingly, one study – which had men and women having sex or masturbating in the lab – found that the increase in prolactin was 400 percent greater following intercourse rather than masturbation, suggesting it is more "physiologically satisfying", according to the study's authors. Prolactin was also thought to play a role for why men can't go for "round 2", however recent research has shown that might not be the case.
As for why this is useful from an evolutionary standpoint, we don't really know for sure. The cocktail of hormones released after orgasm is thought to have a role in bonding with your partner, and it's possible that the feelings of sleepiness are more of a side effect. Others have suggested it could be to do with preventing women – whom studies have found can get just as sleepy as men after sex, particularly after orgasm – from seeking other partners, which is significantly more difficult to do when asleep. Both these theories, of course, would need exploring with more research.
As for why the stereotype is of men falling asleep, rather than men and women, that may be more to do with the orgasm gap.
"[The sleep-inducing hormones] seem to be secreted in equal amounts in men and women," physicians and writers Mark Leyner and Billy Goldberg wrote in their book Why Do Men Fall Asleep After Sex?: More Questions You'd Only Ask a Doctor After Your Third Whiskey Sour, "but we all know who orgasms more frequently".