Found this from one of my friend’s blog.

The unscientific reason:

me: yah. why so men fall asleepp after sex?
kuku: coz they lose their sex drive
me: so if men didn’t have their sex drive, they’d be sleeping all the time?
kuku: well. they don’t really sleep. they just lose interest..
me: so men’s sex drive drives them. without it, they’d just be sleep all the time? so men are driven only by their sex drives?
kuku: that happens to different extents in different men.

so i conclude. if men weren’t motivated by sex.. they’d be asleep all the time.. so everything they do, it’s just their sex drive talking..

The real reason:

At orgasm, both men and women release the hormone oxytocin. This is the same chemical the body releases during breastfeeding, which is responsible for putting babies to sleep as they suckle the breast. Why would a grown-up boy be any different? He wouldn’t. Oxytocin is also known as the chemical that precipitates “bonding” between mother and infant during breastfeeding and may be what makes women want to cuddle after sex. In men, it may show up simply as the “urge to merge.”… more

Now I know…