So why is it that men always fall asleep straight after sex?

Well, other than the idea that now that they’ve got what they wanted they’re not really interested in anything else?

This article tries to work it out and fails slightly on the evolutionary arguments. Her point is that men have a recharge time and since in that recharge time they can’t be off impregnating someone else, why not go to sleep?

Well, yes, but this comes up against an entirely different evolutionary argument made years ago by Desmond Morris (he of “Naked Ape” fame). If sleep after sex was for some evolutionary reason then we would expect it to be women who went to sleep. After all, if she’s asleep then her mate doesn’t have to worry that she’s off getting impregnated by someone else. He also posited that the female orgasm came from the same reasoning: that feeling of languorous satisfaction would achieve the same purpose.

However, the article is on much stronger ground when it looks at the bio-chemical reasons as to why men always fall asleep after sex.

Then there is the biochemistry of the orgasm itself. Research shows that during ejaculation, men 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 naturally higher during sleep, and animals injected with the chemical become tired immediately. This suggests a strong link between prolactin and sleep, so it’s likely that the hormone’s release during orgasm causes men to feel sleepy.

We do need to note though that not everything that happens in a human being happens for a purpose. There’s all sorts of things that go on simply because they do: usually as a side product of something else which is there for some evolutionary reason. In this view the prolactin release is there to heighten the physical feelings and thus the time devoted to pursuing the activity. It might also be to stop men chasing the next woman until the sperm factories are ready again (for in one sense sex without the possibility of procreation is an evolutionary waste, effort expended for no return). But the sleepiness might well be just a side effect.

So the reason that men fall asleep straight after sex might simply be, well, because they do.

But don’t worry ladies, it is entirely natural: it’s not you they’re falling asleep on.