I've been taking moderate sized doses of melatonin for a few months now.

Here is something I posted in another thread:

5mg: More comfortable sleep. No changes in dreams.
10mg: Good Sleep. More vivid dreams. Dream characters are more hostile.
15mg: Fantastic sleep. Less vivid dreams but greater intensity. Dream characters are very violent and will often attack if disturbed.
20mg: Heavy sleep. Slept for 45 minutes longer than usual. Vivid dreams (about the 10mg level). No lucidity whatsoever. Reguar dream signs are ineffective. Dream characters are not violent at all.
25mg: Heavy Sleep. No lucidity. Little to no dreaming.
For the past few weeks I have noticed that I've become desensitized to the effects of melatonin. Where I was taking 20-25mg of melatonin per night, I am now taking 5mg maybe once a week, usually not even that.

I first noticed the effects of melatonin withdrawal when I stopped taking melatonin for a couple of days. The first night that I didn't take it I had a lucid dream that lasted around 15 minutes in dream time. Last night I hadn't taken any melatonin for a few days and I had 2 lucid dreams that lasted around 1 minute each, and another where I tried to phase into the dream but didn't quite make it in.

I'm not sure if it was the melatonin or if it was just a coincidence that I had the LD's. What do you think?