There is no single, definitive scientific explanation for this age-old conundrum. But if you think about it long enough, the sex–sleep connection makes sense, particularly when you consider that many men have their first orgasms while unconscious. Wet dreams, nocturnal tumescence, morning erections…When all is said and done, we may not be much better than praying mantises: The males keep copulating even after they’re decapitated by their lovers.

After orgasm, both men and women release the chemicals oxytocin, prolactin, gamma amino butyric acid (GABA), and endorphins. Each of these contributes to that roll-over-and-snore feeling. The hormone oxytocin is known to have several effects, including establishment of maternal behavior, stimulation of uterine smooth-muscle contraction at birth, and stimulation of milk ejection (milk letdown). It is also referred to as the “cuddling hormone” because it tends to elicit the need to be close and bond. In one study, oxytocin was shown to inhibit male sexual behavior in prairie voles. Maybe it’s the oxytocin that makes us feel satiated and rested.

Prolactin is another player in the sex/sleep saga. It is produced in the pituitary gland, and its best-known function is the stimulation of milk production. Prolactin is believed to relieve sexual arousal after orgasm and take your mind off sex. Levels of prolactin rise during sleep, and some patients with prolactin-secreting tumors report sleepiness. So prolactin seems like it may be a culprit.

GABA and endorphins also both have a calming effect and may make you pass out after sex. So why does the postcoital snooze seem to be so much more a man thing?

This may come as quite a shock to some of you self-annointed Don Juans, but whereas men are known to ejaculate in various semicomatose states, such as during prostate exams and while thumbing through dog-eared issues of National Geographic magazine, women—believe it or not!—don’t always have orgasms during sex, and that keeps them from producing all those other hormones. So, if performance is a problem, don’t miss our five easy ways to make sex last (much) longer.

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