How much caffeine is in your blood right now?

Caffeine clears the body on a half-life, which is the time it takes for your system to break down half of what is left. For most healthy adults that half-life averages about 5 to 6 hours. So if you take in a dose now, roughly half of it is still around 5 to 6 hours later, half of that remains after another 5 to 6 hours, and so on. That is why an afternoon coffee can still be working against your sleep at midnight.

Because the decay is exponential rather than a hard cutoff, caffeine is never instantly gone. As a rule of thumb it takes about five half-lives for a dose to drop to a negligible level, which lands most people somewhere around a full day for complete clearance. While the caffeine is active it blocks adenosine, the molecule that builds up through the day and makes you feel sleepy, which is exactly why a late dose keeps you wired. How fast you clear it comes down to your CYP1A2 liver enzyme, so a slow metabolizer feels the same espresso shot for far longer than a fast one, and that genetic difference is what caffeine sensitivity really describes. This caffeine half-life calculator models that exponential decay directly: it tracks how much caffeine is left in your bloodstream minute by minute, adjusts the half-life for your sensitivity, age, and pregnancy, and predicts when you will fall below the threshold that affects sleep.