The interesting thing is to try and understand why it happens. My guess is that it benefits the survival of the woman. Way back in the long long ago not all sex was consensual. The 'bad touch' sex did the trick of getting the woman pregnant, but getting her away from that caveman was the problem. If the guy always got drowsy then the woman always got an opportunity to sneak away.
Think of life as person. It has it's own agenda and it knows what works to make babies. It doesn't care if men or women get hurt along the way. Life wants to continue life. It wants to make babies and it'll roll right over anything that gets in its way.
My 2 cents Originally Posted by Longermonger
How many have gotten away on you while you slept?