Melatonin is useful for people who can't get to sleep.

It's not so useful for people who wake up during the night, and it's not at all useful for people who wake early.

Your lack of understanding of insomnia is causing you to massively over-simplify, which is probably really fucking annoying to people who've lived with insomnia for years and gone through good quality medical and psychological treatments.

melatonin is one of those "low hanging fruit" solutions people always give for insomnia. And 99% of the time, it never works. At least for me. If only it was that easy.