Tangentially I'm interested to know whether taking melatonin for long periods of time (years) reduces its effectiveness. I've been taking it for nearly a year but I've heard stories about how you need to keep increasing the dose to get the same effect.
Anecdotally, I am taking it for years and haven't noticed any reduced effectiveness. Effectiveness increased when I reduced the dose to 0.3 mg. and started taking it several hours before sleep.
https://slatestarcodex.com/2018/07/10/melatonin-much-more-th... is pretty much a required reading on melatonin, as well as https://www.gwern.net/Melatonin. Neither discuss tolerance at length unfortunately.