Well... ive noticed that after smoking indicas (richer in CBD cannabidiol), that are the relaxing, mellowing strains, i sleep more heavily, and seem to dream less (or to remember less dreams)... as dreaming is related to the REM sleep (if im not mistaken we only dream during the REM sleep), i would risk to say that the CBD is the substance that causes it. But its only a guess, and i may be completly wrong, so dont take my oppinion more seriously than it deserve.