Everyone produces melatonin naturally. If you're too low, the supplement will probably work well for you, if not, it won't do a thing and may cause weird jittery feelings and even sleeplessness.
Any kind of light will cause your body to immediately absorb a melatonin supplement, rendering it useless even if you might, otherwise, benefit from it. After you take it, you really should sit in a very dim room in order for it to work properly. Even watching TV can weaken the effect for some people. In the reverse, if you overdose and feel strange, just head to your bathroom and turn on the bright lights and the problem will resolve quickly.
My naturopath suggested a range of 1-9 milligrams for me. She suggested up to 20 milligrams for my daughter who is severely low.
One trick that I learned that is far from natural, but soooo effective is to take melatonin in the form of sugary gummies. Something about the sugar/fructose base helps your body absorb more effectively, and I swear by it compared to other forms I've tried. LOL, I brush my teeth in the dark after I've taken them.