Speculation: I'd expect a huge selection effect w.r.t. caffeine half-life. If your physiology yields a really long caffeine half-life, you won't drink as much b

Speculation: I'd expect a huge selection effect w.r.t. caffeine half-life. If your physiology yields a really long caffeine half-life, you won't drink as much because it keeps you from sleeping. These people will show up in studies as "not habitual users" Not speculation *up to* 9 hours. Your Mileage Will Vary. My point was for people who aren't sleeping well, who should be extra aware of the stimulants they're taking and how long they can be active. 4-6 hours is also pretty long, which means there aren't very many lambdas between that afternoon cup and when you're trying to go to bed.