Immediate-release melatonin has a short elimination half-life of about 20 to 50 minutes. Meaning if you wake up too early or halfway through sleep, you might as well take it then.

Higher doses do not appear to result in a stronger effect but instead appear to cause drowsiness for a longer period of time.

It’s safe to take melatonin everyday for 12 months. Might not be studied sufficiently to know beyond that.

It’s generally makes people fall asleep 6 minutes faster, but doesn’t change overall sleeping time.

It’s useful for a bunch of other health problems, read the article to see if it applies to you.

https://en.m.wikipedia.org/wiki/Melatonin_(supplement/medication)