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Once I learned the half-life of caffeine is 5-9 hours, I started a rule where I only had caffeinated coffee before noon, and decaf after. Night and day difference (heh) in my sleep quality.

Perhaps 12pm didn't work for you because caffeine stuck around longer in your body compared to mine. Did you ever try an earlier cutoff time?

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I don't think there is a hard rule around half life times, individuals metabolise it differently. Its also not just about the caffeine leaving your system, but the impact of cortisol being increased along with any other stresses you might have on any given day.

I honestly prefer not drinking it all now. I operate better, feel more relaxed and its nice not having an addictive dependence to plan around (although its a less problematic one as coffee addiction is well catered towards). It's one of the best things I have ever done to improve my life, why would I want to try and sneak a bit back in.

The effect of caffeine is controlled by several genes. One is the CYP1A2 gene that encodes for a liver enzyme critical for the metabolism of caffeine. Another is the AHR gene, which controls when and how the CYP1A2 gene is switched on and off. The general population have variants of these genes and so experience different response to similar doses.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3777290/

Same here, except a 3pm cutoff has been adequate. That said, I usually have just one cup a day, and many (most?) other coffee drinkers seem to have more per day than I do. How long it stays in the body is about both how much coffee _and_ how late.

If I were a heavy enough coffee drinker, maybe I'd even have to limit myself to before 10am only, to get sleep improvements?

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