My son's pediatrician said long term melatonin can be done one month on, one month off. I think it was a placebo effect for my son. When he took melatonin, he believed he would fall asleep, so he would do sleep hygiene and not stim. Otherwise, he stims for about 2 hours before he can sleep. Meditation helps him relax and sleep when he remembers to use it. He has ADHD too.
Re: placebo effect, you can buy sugar pills online. So if you're dispensing melatonin, you can use te placebo effect by still giving your child a pill on the days he is on a melatonin break