As with a lot of medicine the dosage matters a lot. I will take .5 mg of melatonin when going to bed earlier than usual and the effect is fairly subtle. But it's commonly sold in 10mg doses which seems very high.
I take 1mg of melatonin every night, and it does immediately make me sleepy. I used to buy 10mg at the drugstore and it just makes me feel like shit for the next day.
The 10mg makes you feel bad because it’s still in your system the next day. At the recommended (lower) doses, your body fully clears it out by morning, so you feel alert and also have much lower risk of long term effects.
https://slatestarcodex.com/2018/07/10/melatonin-much-more-th...
That sounds quite magical. Wouldn't it be more likely that the uptake characteristics were very different for the two variants? Not sure if possible but maybe the 10mg variant were still being digested the day after so it would make you sleepy then instead of during the night?
After some digging here are some interesting discoveries related to melatonin and doxylamine:
https://www.reddit.com/r/insomnia/comments/spoaec/anyone_hav...
https://www.reddit.com/r/insomnia/comments/spoaec/anyone_hav...