@Anti: Well it's not connected with being drunk or not. It's just that alcohol in general causes dehydratation (nothing severe though) and thus you get really thirsty especially when you wake up afterwards. If you're too thirsty however, in most cases you'll wake up.
Hmm... I know that alcohol cause dehydration but in my case, I never woke up just because I needed a drink ...
It can be weird o.O
But only the person's body can understand well I guess