Time is relative in both the waking and dream world.
Interesting fact, if you become sleep deprived enough or sleep too much you lose the ability to distinguish when you are awake or asleep and fall in and out of dreams.
Another interesting theory that is out there is the subconscious is collective for our race and when you dream your connected to everyone else who is sleeping. So with that in mind, here's a question. If you have a committed relationship and you have a dream about sleeping with someone else, have you cheated on your partner?