Why do guys get sleepy after sex?

For many women, the correlation between sex and snoring is one of those annoying facts of life: no matter when passionate encounters occur, men always seem to fall asleep immediately afterwards. Dave Zinczenko, the author of Men, Love and Sex: The Complete User Guide For Women, explained the phenomenon to Huffington Post writer Arianna Huffington this way: “Men go to sleep because women don’t turn into a pizza.”

I doubt I am ever going to become a pizza, and I’ll never have the foresight to order one beforehand. So in lieu of a cure, a better explanation will have to do. Women, too, often feel sleepy after sex. What is it, then, that spirals us into the land of nod? And could it actually be a good thing?

First, the obvious reasons for sex’s somnolent sway: the act frequently takes place at night, in a bed, and is, after all, physically exhausting (often more so for the man than the woman, although this certainly varies). So when sex is over, it’s natural to feel sleepy.

Secondly, research using positron emission tomography (PET) scans has shown that activity in the amygdala, the brain area that controls fear and anxiety, decreases prior to orgasm. University of Groningen doctor Gert Holstege, who led the study, told the London Times that “letting go of all fear and anxiety might be the most important thing, even necessary, to have an orgasm." Doing so, of course, tends to be relaxing and might explain the tendency to snooze.

Then there is the biochemistry of the orgasm itself. Research shows that during orgasm, we release a cocktail of brain chemicals, including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide (NO), and the hormone prolactin. The release of prolactin is linked to the feeling of sexual satisfaction, and it also mediates the “recovery time” that men are well aware of—the time a guy must wait before “giving it another go.” Studies have also shown that men deficient in prolactin have faster recovery times.

Prolactin levels are also naturally higher during sleep, suggesting a link between the two. It's possible that the hormone’s release during orgasm leads to drowsiness.

(Side note: prolactin also explains why we are sleepier after intercourse than after masturbation. For unknown reasons, intercourse orgasms release four times more prolactin than masturbatory orgasms, according to a recent study in the Journal of Biological Psychology.)

Oxytocin and vasopressin, two other chemicals released during orgasm, are also associated with sleep. Their release frequently accompanies that of melatonin, the primary hormone that regulates our body clocks. Oxytocin is also thought to reduce stress levels, which again could lead to relaxation and sleepiness.

What about the evolutionary reasons for post-sex sleepiness? This is trickier to explain, and no one really knows. Evolutionarily speaking, a man’s primary goal is to produce as many offspring as possible, and sleeping doesn’t exactly help in his quest. But perhaps since he cannot immediately run off with another woman anyway—damn that recovery time!—re-energizing himself via sleep may be the best use of his time.

And women often fall asleep with the men anyway (or use it for some key cuddling time), which could aid with conception (you know, gravity and all that). And when the two wake up naked together, they just might be ready to go again.

It’s also possible that sleepiness is just a “side effect” associated with a more evolutionarily important reason for the release of oxytocin and vasopressin. In addition to being associated with sleep, both chemicals are also intimately involved in what is called “pair bonding,” the social attachment human mates commonly share. The release of these brain chemicals during orgasm heightens feelings of bonding and trust between sexual partners, which may partially explain the link between sex and emotional attachment. This bond is favorable should the couple have a baby, as cooperative child rearing maximizes the young one’s chances for survival.

The bottom line is this: there are many potential biochemical and evolutionary reasons for post-sex sleepiness, some direct and some indirect—but no one has yet pinpointed the exact causes. One thing, however, is certain: we all better get used to it, because it doesn’t look likely to change anytime soon.

I will leave you frustrated American women with one final thought: if you are upset at the ubiquity of the post-sex snoring phenomenon in your men, remember that things could be a lot worse. A recent survey of 10,000 English men revealed that 48 percent actually fall asleep during sex.

Talk about coitus interruptus!

A version of this story was first published here on September 25, 2006.