Caffeine

Caffeine half-life: how long caffeine really lasts

Sourced explainer · By Jeffrey Dietrich, PhD · ~6 min read

The reason a 3 p.m. coffee can follow you to midnight isn't that caffeine is unusually powerful — it's that it leaves slowly. The single most useful number for understanding how long caffeine lasts is its half-life: about five hours. Once you know what that means, the rest of caffeine's behavior stops being mysterious.

What “half-life” actually means

A drug's half-life is the time it takes your body to clear half of what's present. It isn't the time until the drug is gone — it's the time until there's half as much. That distinction matters, because clearance is exponential, not linear: each half-life removes half of whatever remains, so the drug fades in a long, tapering tail rather than dropping off a cliff. For caffeine, that tail is exactly why an afternoon cup can still be working at bedtime.

Caffeine's half-life: about 5 hours

In healthy adults, caffeine's plasma half-life averages roughly five hours, with a commonly reported range of about 3 to 7 hours — and wider tails in either direction depending on the person.¹ Five hours after a coffee, half the caffeine is still circulating. Ten hours after, a quarter. The dose doesn't vanish on schedule; it lingers and overlaps with the next cup.

The decay table: watch a 100 mg cup fade

Using a five-hour half-life, here's what's left of a single 100 mg dose — a modest cup of coffee — as the hours pass:

Two things jump out. First, a quarter of the dose is still active ten hours later — so a noon coffee is meaningfully present at 10 p.m. Second, full clearance takes close to a day: it's roughly five half-lives before caffeine is effectively gone. Drink a second and third cup and they don't reset the clock — they stack on top of what's already there.

Why the half-life matters for sleep

Caffeine keeps you alert by blocking adenosine, the molecule that builds your sense of sleep pressure through the day. The long half-life means that block is still partly in place hours after you've stopped noticing the lift — quietly trimming your deep, restorative sleep even when you fall asleep without trouble.² That's the trap: feeling fine at bedtime isn't evidence the caffeine has cleared. For the full picture, see caffeine and sleep.

What speeds up — or slows down — clearance

That five-hour average hides enormous person-to-person variation. The main driver is a single liver enzyme, CYP1A2, whose activity is shaped by genetics, age, hormones, and habits.¹ The big movers:

Pregnancy is the most striking: by the third trimester, caffeine's half-life can rise to around 10 to 11 hours — longer in some reports — so a single cup behaves like several.³ Smoking pushes the other way, which is also why people who quit often find coffee suddenly hits harder.

Fast vs. slow metabolizers

Because of CYP1A2 variation, two people can drink the identical cup and live in different worlds. A fast metabolizer clears caffeine quickly, tolerates an afternoon coffee, and may barely notice it by evening. A slow metabolizer keeps the same dose circulating for many extra hours — the lift overstays, sleep suffers, and the "stop by 2 p.m." rule of thumb is far too generous.¹ If caffeine seems to hit you hard and linger, you're likely on the slow end, and your personal cutoff should be earlier than the generic advice. For what the molecule is doing in the meantime, see what caffeine does to your body.

The practical cutoff: when to stop

Translate the half-life into a rule and it comes out simple: stop caffeine 8 to 10 hours before bed for an average metabolizer, and treat that as a floor — not a target — if you're slow, older, pregnant, or simply sensitive.² Front-load your intake earlier in the day, keep the total moderate, and don't chase a poor night with a bigger afternoon dose. The catch is that this advice cuts off exactly the afternoon-and-evening window when many people most want a lift.

Paraxanthine: a shorter tail

When your liver processes caffeine, the main thing it produces is paraxanthine (Px) — the compound your body makes from caffeine, and the one that carries a portion of its alertness. Taken on its own, paraxanthine has been reported to clear faster than caffeine: a half-life of about 3.1 hours versus caffeine's roughly four to five.⁵ A shorter tail is precisely what the evening problem calls for — the lift when you want it, less of it left at midnight. That human evidence so far is limited and comes from small, industry-funded studies, so it is best read as emerging. How that works is covered in paraxanthine vs. caffeine.

* These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Content on this page is informational and not a substitute for medical advice; talk to a clinician about caffeine and your health.

Keep going: caffeine, explained · caffeine and sleep · what caffeine does to your body · paraxanthine vs. caffeine