Why do men fall asleep right after sex?

For many women, this must be one of the most annoying facts of men:
"First, the obvious reasons for sex’s somnolent sway: the act frequently takes place at night, in a bed, and is, after all, physically exhausting (often more so for the man than the woman, although this certainly varies). So when sex is over, it’s natural for a guy to feel sleepy.

Secondly, research using positron emission tomography (PET) scans has shown that in order for a person to reach orgasm, a primary requirement is to let go of “all fear and anxiety.” Doing so also tends to be relaxing and might explain the tendency to snooze.

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."

Read the book