Firstly, I assume you mean down regulated and not up regulated

Secondly, this is the same as practically every receptor binding drug consumed at high doses

There is no reason to take melatonin at high doses, in fact, the dose response curve drops off and make high dose melatonin less effective than low dose melatonin

1mg or less slow release seems to be the ideal in research, and research has cleared the safety for up to a year of continuous use at which point there might be cardiovascular concerns in severe insomniacs - but the research is poor quality and full of holes to the point where the authors themselves are dubious of the correlation and call for more research