A dreaming person has the pattern of someone who is wide awake and alert. The latter happens without any REM cum EEG fanfare. The relationship is very much like the one that exists between intuition and formalism, aesthetics and scientific discipline, feeling and thinking, mentally creating and committing one's creation to a medium.
Some birds do it, and some reptiles as well.
The uterus contracts and the muscles at the base of the tongue enjoy a relaxation in electrical activity. These facts would indicate that dreaming is a very primordial activity. It is essential to survival. And the mentally disturbed and the mentally retarded dream as much as the normal do. Such a model can be innate and genetic in very simple forms of life because the amount of information that needs to be incorporated is limited.