Hope you don't mind I've copied my thread over from Children's Health - might get better info here! 
DS1 is autistic and has always struggled to sleep. He's 8yo, and has been on prescribed Melatonin for a year or so, at fairly low doses.
Before it was prescribed by his consultant we had been ordering it online in gummy form for the previous 6 months, and it had been helping him settle.
His sleep issues are primarily settling, and night terrors. He can't get to sleep at all, unless exhausted or recently stressed - it has been suggested this is related to his adrenaline levels, as if he gets upset (ie has an adrenaline rush) he'll often conk out straight after.
He is currently prescribed 3mg Melatonin (fast release), and 4mg Circadin (slow release).
The Circadin seems to have stopped his night terrors, but he isn't getting to sleep til 11/12/1am.
We've tried doubling the Melatonin for a few weeks, which hasn't helped.
He'll read, play in his room, or keep coming down to us for hours on end, he just doesn't get sleepy.
To compound it, the surgery keep sending the pharmacy the wrong repeat item, so when we request the 3mg Melatonin they keep sending the 2mg Circadin (he takes x2 for the correct dose), so we keep running out of the Melatonin while we email back and forth trying to get the correct one, and it's special order so takes a week to come in...
Any advice appreciated, sorry I'm so longwinded.