MIT has a patent on doses less than 1 mg and some(not exactly >1 mg) higher doses can have harmful effects.
See http://www.gwern.net/Melatonin for a good writeup :)
Did not know about the MIT thing. That explains why it seems impossible to find melatonin in reasonable doses. It took me a long time even to find something as low as 1 mg pills here in europe (why would anyone ever want to buy 8 mg melatonin pills??)
No, it can't, not all hormones work that way.