It seems that only small amounts are soluble in water. I have made this lemon juice solution of approx 20 mg melatonin on 200 ml fluid... when I tried 200 mg on the same amount of fluid the greater amount of it kept floating on top. Now I suspect that at least a part dissolved but no more than that. I found that the 20 mg bottle proved to be reliable, that each dose of same quantity had the same effect.

Also I didn't understand something. That if you increase the dose some different effects can occur. That there is no straight line between dose and effects, it's more like a landscape that changes along the road. It does takes out the effectiveness, so instead of falling asleep I keep comfortably in a sort of zone between sleeping and waking going through all sorts of interesting stuff (with 10 to 20 mg).