You know, it's interesting for me to think about this because I remember back when I first started developing DP to a serious level, I would ask friends and such what they thought of the idea that we might all be in a dream. I would read a lot of Des Cartes in an attempt to find some kind of proof that this was "all real" and not some funky-assed dream (as a side-note, my favorite answer a friend gave me to this day was "if this was a dream - I would be in bed with that blonde over there"....she was pretty hot). In the end I realized that Des Cartes was actually funky-assed.
Now, this isn't meant to scare anyone - as it's quite possible those who ask this question now may actually be asking it at a different stage (I'm not saying that you should expect worse 'cause I was making these questions at the beginning of my disorder).
However, the commonality between our awake thoughts and the common-person's asleep thoughts can't be ignored.
Ah well, the important thing to realize, however, is that trying to solve your DP-induced questions won't get you anywhere; trying to understand how it can happen to a biological creature like us, however, does have an answer and is interesting to think about.....which is the nature of my incoherent, rambling, nonsensical, omnidirectional, potentially wasteful, for-God's-sake-someone-put-more-Johnny-Walker-in-my-Caffeine-free-Coke-so-I-can-fall-asleep post.