They are best known for their research on the human sexual response, on which they published a book of the same name in Dasgupta explained that most couples have sex at night, potentially in cool, quiet rooms, likely in a comfortable bed — in other words, the perfect environment for sleeping. You'd Also Like. If you're not sleeping well because of stress or lack of proper resting conditions, you'll be tired, grumpy and irritable and your sex drive is going to be way down. So it'll be nearly impossible for him to fight the urge to hit the hay if you just lie there quietly. Prolactin is believed to relieve sexual arousal after orgasm and take your mind off sex. So maybe move the snuggle party to the couch. So in lieu of a cure, a better explanation will have to do. Ever wonder what it is about sex that makes men want to pass out immediately afterwards? Your message is that it's time for sleep because our mind is used to associate the horizontal position in bed and the lack of light with the opportunity for getting the necessary rest. 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. When that happens, the hormone melatonin ignites his sleep cycle. When prolactin, oxytocin, and melatonin all come together, it's basically the natural version of any serious sleeping pill—and he's in for a fantastic snooze. A recent survey of 10, English men revealed that 48 percent actually fall asleep during sex.