Desire is the important aspect of dream. People want a lot but don't get. Even, he does not know what he want really. On the other hand, dreams of looking at things that are not available in one's dream. On the other hand he know that, What he wants? he can't know that.
people have to sleep for 6 to 8 hours. Otherwise, it is enough for people 2/3 hours for physical rest. The dream of common is as per as sleeping. No required to dream if there is a level of spiritual pursuit.