I take a mix of melatonin, l-theanine, and 5-htp almost every night to go to sleep, and I've definitely found that if I don't take it for a couple days (usually Fri and Sat night) then take it again, my dreams are overly vivid and memorable.

While I'm on it, they're pretty suppressed. I used to take 10mg of melatonin every night and the effects were the same I'd say, so it doesn't seem to matter on dosage.