I never had Terragen dreams (so far), but I can't look at nature like I did, before I put my hands on TG. Sometimes I see something, and I think, this looks like a bad render, or how can I achieve something like this in TG.
I'd love to have a TG dream, and some time ago I read a lot about lucid dreaming (being aware that you're dreaming, when you sleep and being able to control the dream to a certain extent). Maybe one day I'll manage to have a lucid one with TG content.
Btw, the chance to have vivd dreams are good during the REM phases in the early morning.