Melatonin

[Angrboda]

It is possible (but unlikely) that melatonin has little effect on him.
I know people who get the shakes after two cups of coffee, but I drink coffee almost every hour on the hour and never seem to get any effects .. I just like the taste (and yes I sleep well at night!)

It's entirely possible. I wasn't disputing that. It's also possible that it has and he doesn't know it. It's also possible that it hasn't but it will. All these possibilities counsel against taking large doses of melatonin on a regular basis.