Caffeine Decay

Caffeine decay formula

Your body clears caffeine by first-order kinetics, so the math is just C(t) = C₀ · e^(-k·t), with k = ln(2) / t½. In healthy adults the half-life sits somewhere around 3–5 h, with a median near 5 h. Example: drink 200 mg at 18:00 and by 23:00 you are down to roughly 100 mg at 23:00, then around 50 mg at 04:00 when you would rather be asleep. Genetics matter a lot here. The CYP1A2 polymorphism sorts people into fast metabolizers (AA homozygotes, t½ ~3 h) and slow ones (AC/CC, t½ 8 h+). Pregnancy stretches clearance about threefold (t½ 9–13 h), and a newborn can take up to 80 h. The practical takeaway is to cut caffeine at least 6 hours before bedtime.

Applications

Timing a pre-workout dose (3–6 mg/kg about 30–45 min before training). Keeping intake in check during pregnancy (FDA <200 mg/day). Chasing down insomnia by lining up the last dose against the bedtime curve. And riding out withdrawal, where the headache tends to last 3–7 days if you quit cold turkey.

FAQ

Why does coffee not keep me awake? Probably a fast CYP1A2 variant, or tolerance built up from drinking it every day. Either way, your sleep architecture can still take a hit even if you do nod off.

How long until caffeine is fully cleared? Five half-lives knock the dose down to about 3% of what you took. With a 5 h half-life that works out to 25 h.

Does decaf count? It does. Figure 2–15 mg a cup, which is small but not nothing.

Is the model exact? No. It runs on average pharmacokinetics, and your own half-life shifts with genetics, smoking, oral contraceptives and liver function.