Indeed, and the variance on this number is surprisingly large. From [1]: > The mean half-life of caffeine in plasma of healthy individuals is about 5 hours. Ho

Indeed, and the variance on this number is surprisingly large. From [1]: > The mean half-life of caffeine in plasma of healthy individuals is about 5 hours. However, caffeine's elimination half-life may range between 1.5 and 9.5 hours, while the total plasma clearance rate for caffeine is estimated to be 0.078 L/h/kg (Brachtel and Richter, 1992; Busto et al., 1989). This wide range in the plasma mean half-life of caffeine is due to both innate individual variation, and a variety of physiological and environmental characteristics that influence caffeine metabolism (e.g., pregnancy, obesity, use of oral contraceptives, smoking, altitude). On the one end of the scale, it'd be nigh-undetectable within a few hours, and on the other end, significantly present for 24 hours+. I suspect estimating this number is also fairly difficult. I mean, I have some idea of the _outcome_ of what happens if I drink coffee at say 6PM, but I have little way of reversing this into a half-life number. [1] - https://www.ncbi.nlm.nih.gov/books/NBK223808/ https://www.ncbi.nlm.nih.gov/books/NBK223808/