That's something that always got me about people who claim that their dreams are just as realistic as waking life.
The only time my dreams have ever seemed to me to be "realistic" is when I am in them, and I am convinced that I am in reality. However, once I wake up I realize that it was just a dream. Sometimes I'll come the realization while I'm in the dream, although I rarely gain the lucidity to control it.
I guess you could say that this proves that it's all just sensory input and awake/asleep makes no difference to your brain, but I can say this; every day I wake up to the same reality, but every night I dream a very different dream world.