Dreams are really interesting and really complicated in my mind. Most people rule dreams out as conjurings of the mind, but I am of the opinion that the dream state can transport your "mind" to another, higher dimension (call it astral travel or whatever). I don't think that this is necessarily dreaming, it just occurs while you are in the deep state of conciousness where or near where dreams occur. Let me explain, there are 4 different types of conciousness, or 4 different types of waves that one's brain produces during different actions (if you want to correlate these I'm sure you can find a website on your own, but
here is one) according to the EEG machine invented by Hans Berger (hehe):
Beta waves: the brain produces these when you are awake and active. Doing chores and such. 12-15 cycles per second and up.
Alpha waves: 6-14 cpcles per second, this is where daydreaming, nocturnal dreaming and hypnosis occur. Also, if you believe in that stuff (I happen to), psychic experiences may occur here as well.
Theta waves: Approximately 4-8 cycles per second here. This, it is speculated, is where our "emotional experiences" are recorded and where most psychic activity takes place.
Delta waves: These are during unconciousness or deep dreamless sleep, anything less than 4-5 cycles per second.
Now, during sleep one generally quickly through beta, alpha, theta and into delta for a short time (transition takes like 30 minutes, generally, and you would stay int delta for like 90 minutes). Then you would cycle up to theta and alpha where you would dream for awhile. That process would continue through the night. (This information came partially from Psychic Development for Beginners by William W. Hewitt)
I believe that during the time spent in the Theta area, one is able to mentally travel into another, higher dimension. This is supported by quantum mechanics which says that our brain sees much more than our eyes can process (like having a camera with a cloudy lens). So, I would say that the "astral" dimension is right here on earth, but we are unable to see it with our eyes due to their out-of-date ness, but we are able to see it with our mind, which can be "freed" (so to speak) when our physical body is asleep and dormant. (By the way, my quantum mechanics information comes cheifly from memory, but check out the movie What the Bleep Do We Know for an awesome quantum theory time). I also think, that if one were so inclined, they could train themselves to go to this dimension voluntarily rather than in dreams. Because, in theory, all one would need to do to be psychic is to be able to deliberately transport your mind from beta to theta and alpha waves (which takes years of training and obscene amounts of time and isnt nearly as easy as it sounds...which would be that I'm not very good at it what with school and stuff [becuase I know someone will ask]).