I don’t know about melatonin specifically, but some people react differently to chemicals than others. I’ve got a friend who could drink half a can of caffeina

I don’t know about melatonin specifically, but some people react differently to chemicals than others. I’ve got a friend who could drink half a can of caffeinated soda and literally be wired as if he’d taken a recreational upper. I found I slept easier when I took adderall which would have the opposite effect on most folks.