I definitely think that if you're taking them under the tongue the doses they sell them in are much too high. Probably make more sense if you're swallowing them.
Melatonin works great for me as well. I take 5mg. Not sure if my dreams are any more vivid than without it, as I've been taking it every night for years. I think I've always had vivid dreams, though I no longer get recurring nightmares (which I had years prior to taking melatonin; not sure when they ended).
edit: since everybody is asking the same question about habit forming. I am just passing on the information that I got from my doctor. She made this suggestion to me, and it was beneficial to me. Once I had started using melatonin, I would have trouble falling asleep without it. Valarian root didn't cause that issue for me.