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I was under the impression that on average the brain/body needs around 9 days of no caffeine consumption to get back to its natural baseline. I can't seem to find the source of that study right now...

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You could do the math if you look up the half-life of caffeine. Via a quick Google search it looks like it is 4-6 hours. So if I understand it correctly, it's 4-6 hours for 100mg to become 50mg, then another 4-6 for 50mg to become 25mg, etc. So like if all you did was have a single cup of 100mg coffee, then its like 12-18 hours for it to go away. If you're me though, you drink like 3-4 a day. This seems to be about 28 to 42 hours. Withdrawals could take up to 2 weeks in my experience, and are proportional to how heavy my intake was.

That tells you how long before it's out of your system, not how long it takes to reset after.

Effect duration for a psychoactive is related to half-life, but they aren't identical, and 'reset time' varies widely, e.g. MDMA is over in about 4 hours but it takes at minimum of two weeks before one can get a comparable effect again.

You'd have to look at things like the rate of downregulation of the appropriate brain components that caffeine affects.

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