Cafeína Restante (meia-vida 5h)

Caffeine remaining after n hours

Your body clears caffeine through first-order elimination, which the formula C(t) = C₀ · (1/2)^(t / t½) captures, where C₀ is the dose, t is the hours that have passed, and t½ is the half-life. Most healthy adults sit around a 5 hour half-life, though it ranges from 3 to 7 h. Take 200 mg at 18:00 with a 5 h half-life and you still have roughly 100 mg at 23:00, about 50 mg at 04:00, and ~25 mg by 09:00. Biology moves that half-life around a lot. Pregnancy stretches it to 9–13 h, clearing caffeine three times slower; oral contraceptives roughly double it; smoking speeds it up to 3–4 h through CYP1A2 induction; and a CYP1A2 polymorphism (AA vs AC/CC) separates adults into fast and slow metabolizers. Newborns need ~80 hours to clear a dose because their liver enzymes haven't matured yet, and kids catch up to adult rates around puberty.

Practical uses

Time a pre-workout dose properly (plasma peaks around 45–60 min, with the ergogenic window running 1–3 h), check whether that afternoon coffee will still be hanging around at bedtime (aim for under 50 mg when you fall asleep), figure out what's feeding your insomnia, or pace anti-fatigue doses across a long drive so you never spike too high. The sneaky problem is stacking: drink more before the last dose has cleared and your blood level climbs into jittery, heart-racing territory before you realize what happened.

FAQ

How do I know my half-life? Short of a genetic test, go with 5 h. Use 4 h if you smoke, or 9–13 h if you're pregnant.

When is caffeine "fully gone"? After roughly 5 half-lives, which is about 25 h for an average adult, only ~3% is left, so call it zero.

Does tolerance change the math? No. Tolerance affects how you feel, not how quickly your liver works. Someone who drinks coffee every day still has 100 mg in their blood at hour 5.

Why does coffee hit harder on an empty stomach? It absorbs faster, peaking in around 30 min instead of the 60–90 min you'd see with food. The total caffeine doesn't change.