For some people, me included, melatonin can cause an uptick in very real feeling and (depending on mental state) very weird dreams. Do be mindful of that. On the other hand if you're practiced at lucid dreaming this can be a real trip.
If you are sleep deprived, anything that helps you sleep better can result in an uptick in very vivid, strange dreams. It is a common side effect of quitting caffeine and any number of other things.
Anything that suddenly and dramatically improves your sleep quality can potentially have this effect.
I've found that the "weird dream" side effect is very dose responsive. Most melatonin supplements I see include doses much higher than are required, in the 3-5 mg range. Very small doses (as the OP mentioned, in the 200-400 microgram range) should be plenty as a sleep aid and (for me anyway) avoid the enhanced dreams.
I will sometimes take 5 mg when I wake up in the middle of the night in an attempt to induce lucid dreaming, although since I've fallen off of my dream journal my LD abilities are diminished (even with the melatonin).
