dont quote me on this
but i read once, i think on erowid.org im not sure, but i read that cannabinoids prevent your mind from sustaining 4th stage of sleep. I read that when u enter 4th stage sleep, you "bounce" back into third and forth, and you dont get a "complete" nights rest as a result. This also attributes to why many pot smokers say they have no dreams/cant remember dreams. For about 5 years I diddnt have a single dream. Then i went on probation (and couldnt smoke herb) and was having the most vivid colorful dreams I ever had, and I had totally forgotton what dreams were!