Next time your wife or girlfriend (or paid companion) complains that you fall asleep right after sex instead of engaging in hours of after-glow pillow talk, tell her it's not your fault -- and you can back it up with science.

French researchers who studied the brain scans of men found that after orgasm, the cerebral cortex -- otherwise known as the "thinking" part of the brain -- pretty much shuts down.

That's followed by two other areas, the cingulate cortex and amygdala, telling the rest of the brain to disengage because an interest in sex is no longer required. And without that, why bother with all the pointless jibber-jabber?

But wait, there's more: once all the above happens, the brain is then awash in oxytocin and serotonin, nature’s very own knock-out hormones. The end result? Zzzzzzzzz.

"As frustrating as it is for most women that their male partners just roll over and fall asleep after sex, men aren't entirely to blame," said Dr. Neil Stanley, director of sleep at the University of Surrey in the UK. "While often seen as just a poor excuse, there are scientific reasons why men feel tired after sex."

But since the same study also found that almost half of men have actually fallen asleep during sex, you'll forgive the ladies if they're not entirely convinced.