We started melatonin on ds about 4 months after a year of trying to fix bedtime. We tried all all the stuff mylovely suggested. Ds didn't fight bedtime - just couldn't fall asleep. He'd lay there in bed nicely trying for the better part of an hour before wanting us to help him and we'd rock him another hour. If we rocked from the beginning we rocked hom for 2 hours. He was so tired each day. If we tried starting earlier he just took longer to go to sleep. If we tried later, he still took 2 hours and just got less sleep.
Ds also has restless leg syndrome so we dealt with that first (in hopes it would also help bedtime). When we'd dealt with the restless stuff (not made it go away but significantly better) he slept an extra hour in the morning but we made no dent in bedtime.
With our pedi's blessing we give ds 1mg meletonin every night. Every couple weeks we try not using to see if he does better. Our pedi said he was 100% comfortable with us doing that low a dose nightly for ds. He did want us to come talk to him if we felt 1mg wasn't doing the trick anymore. He'd want to possibly run some tests to rule out other issues if we needed to bump up the doseage. He felt ds was a good canidate for a sleep aid of some sort and of all the options he felt melatonin was the best choice for us. We initially tried melatonin without talking with our pedi and it was amazing - we gave it to him before we started our routine and about 20 minutes after we turned lights out he was asleep (our bedtime routine takes about 20 minutes). Ds now is rested each morning. His temper tantrums have significantly gone down. His behaviour is 1000% better. Between fixing his restless leg stuff and the melatonin he's getting a good 2-3 extra hours of sleep now and it has made all the difference in him.