That's true, but melatonin is probably not a very representative example, (unless you're only talking about sleep) as its mechanism is really idiosyncratic. It

That's true, but melatonin is probably not a very representative example, (unless you're only talking about sleep) as its mechanism is really idiosyncratic. It seems (from [1]) that the body has a day-clock which is synchronised to the actual day by the release of melatonin. Taking melatonin (at the right time) reinforces the signal; taking too much swamps it completely. It seems unlikely that many other body chemicals are part of the signal chain of a biological PLL (phase-locked loop). [1] https://slatestarcodex.com/2018/07/10/melatonin-much-more-than-you-wanted-to-know/ https://slatestarcodex.com/2018/07/10/melatonin-much-more-th...