Melatonin is one of those things that may suppress REM sleep. When the melatonin in your system wears off, you get a rebound in REM sleep that might make dreams more vivid. In one case, I had a family member who took melatonin on his doctor's recommendation during a stressful period. He actually woke up screaming from the nightmares he had when REM sleep resumed and refused to use it ever again.