[pedalpete]

Can you point to a source about the body down-regulating melatonin?

I also thought this was the case, but everything I've seen suggests that taking melatonin does not alter the natural production of melatonin.

You are correct about everything else though.

[a2dam]

It doesn't. There's some minor evidence, but it's very quickly reversed.

[crdrost]

I mean it's understudied, but at the very least you have [1], children given high doses daily of melatonin developed delayed sleep/wake cycles when measured by DLMO (the time of day that your endogenous melatonin starts to rise) and that “Nearly all children who temporarily discontinued melatonin experienced a delay in sleep onset time,” both of which strongly suggest downregulation is happening. (Usually endogenous melatonin skews earlier with melatonin supplementation, see [2].)

Similar inefficacies have been seen clinically e.g. in [3] and are (caution, anecdata) widespread on the internet, with "melatonin doesn't work" being a popular search term with tons of articles about it. An honest to goodness test seems to have been done at [4] where they made sleep disturbance symptoms "disappear" by resuming treatment at a lower dosage, but instead of blaming the neurons they are blaming the liver, saying that it got overloaded and couldn't clear melatonin out of the bloodstream anymore in some patients—I just want to include that as a plausible alternative explanation so that you don't take my words as gospel truth or anything. I’m trained as a physicist, not a physician, and there is this meme of people with physics degrees thinking that everybody else’s field is their expertise and like I want to be deliberately self conscious about my limitations here.

[1]: PDF Warning: https://www.herbogeminis.com/revista/IMG/pdf/melatonin-adhd....

[2]: https://academic.oup.com/sleep/article-abstract/33/12/1605/2...

[3]: https://www.psychiatrist.com/jcp/effects-exogenous-melatonin...

[4]: https://pubmed.ncbi.nlm.nih.gov/20576063/