why do guys get sleepy after sex?

After reading some of the ahem, “male empowering” posts on UM as of late you might think the answer is simply “Because they’re men.  They do as they please.” While this certainly may be the case, there’s apparently a few other factors involved.

Live Science contends both chemistry, and evolution play a role in post coitus nap time.

Research shows that during ejaculation, men release a cocktail of brain chemicals, including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide (NO), and the hormone prolactin.

This mix of chemicals is a potent sedative.

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.

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.

From an evolutionary perspective, it makes perfect sense to sleep after intercourse.

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.

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.

Read more here.

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