Dreams are sometimes effects of what happened in the fast and sometimes not even related to us.
"Probably the most general conclusion to be reached from a wide variety of disparate stimuli employed and analyses undertaken is that dreams are relatively autonomous, or 'isolated,' mental phenomena, in that they are not readily susceptible to either induction or modification by immediate presleep manipulation, at least those within the realm of possibility in ethical human experimentation."
http://dreamresearch.net/TSSOD/chapter1.html