Melatonin in the US is frequently found in 5 or 10 mg doses, which is a huge amount and can cause adverse reactions in people. A better dose is 0.3mg (300 mcg) or 0.5mg (500 mcg).
That explains a lot. I have a bottle of 5mg pills and I found out I could still feel the effects when I would take a tiny “nibble” of the pill. So over the course of a bad sleep week I would slowly consume a single pill. This bottle is a lifetime supply, assuming it doesn’t degrade.
It seems to be particularly bad with Melatonin, because a lot of people do really badly with big doses. A 3mg dose makes me more exhausted than I've ever felt before melatonin. I tried A couple of times. Never again. I wrote it off until someone pointed out this isn't unusual with doses that high. 1mg dose is fine. I can't image how I'd have felt with a higher dose.
Also, a supplement being labled "500 micrograms" has no requirement to be correct. The FDA has handwaved away the entire industry, and broadly there is zero quality control for actual active ingredients. Many supplements will claim that they've been tested, but there's no legal requirement to do so, the labs are usually owned by supplement makers, and there's no penalty for lying about any testing as fraudulent marketing claims largely go unpunished because the US judicial branch chooses to interpret the law stupidly conservatively
