I experiemented with different doses (of a normal release kind of melatonin) in the past and found out that raising doses higher then 1mg did nothing more for me (I experimented up to 3mg doses). So I was on just 1mg for a while which did really help my inability to get to sleep issue about 2 thirds or more of the time. I was so greatful for the help the melatonin gave me. I was on it for several months but then lost the doctor who used to prescribe it so couldnt get a script for it anymore.
Recently I managed to get melatonin again (this time it has been slow release melatonin) but found it stopped having any positive affects at all after the first couple of weeks. I had been taking my previous dose of 1mg. (I wonder if part the reason it isnt working as well for me is that my previous one wasnt a slow release one?).
Ive just doubled this dose to 2mg and now this dose which previously had no extra benefit for me, made it work (only a bit thou so I still have bad issues) again. Its currently bringing the start of my sleep cycle back from 6.30am to 4.30am when its at its worst.
......
so it seems that melatonin dosage can really vary even in individuals at different times. So it will have to be something you do trials with different dosages of 1-3mg to see.
best luck
ps also consider what kind of sleep issues you have. Melatonin is best for sleep reversal (out of sync sleep cycle) kind of issues and dont work so well for other kinds of insomina. Its more successfully used for those of us who have "Delayed sleep phase syndrome" (DSPS or DSPD)
http://en.wikipedia.org/wiki/Delayed_sleep_phase_syndrome and for those who have "Non-24-hour sleep-wake syndrome"
http://en.wikipedia.org/wiki/Non-24-hour_sleep-wake_syndrome (my specialists are saying I have one of those).
With delayed sleep cycle issues .. melatonin (by studies) can bring ones sleep cycle backwards 30mins to a couple of hrs.