Receptors saturate at around 1000  pmol/L 

Which is the equivalent of like 0.000002mg 

High dosages of melatonin is like pissing into the wind

0.3mg is more than enough

The only reason the dosages are much higher is because it's taken into account bioavailability is very poor. Only 10% of melatonin is absorbed through bloodstream because of first past metabolism in liver