That's the problem, much of the stuff they sell over the counter is too high, it's not regulated medically so they peddle the idea that higher doses are more effective when there is evidence the opposite is the case.

Melatonin and Sleep | The BMJ

https://www.huffingtonpost.com/van-w...b_8855998.html

Edit no I don't disapprove of melatonin, I use it myself as I said, my doctor was originally also trying to raise the dose thinking it would make it more effective and it just made things worse, then we found the research (above) that showed we needed to go lower and that worked better

You asked for why it might be making you drowsy in mornings too, this is a possible answer