How long does caffeine last?

Caffeine works on three different clocks, and most confusion about it comes from mixing them up.

• It arrives in 15 to 20 minutes and peaks in the blood at about 45 minutes.
• Half of it is gone in 5.7 hours. That is the number that decides your evening.
• The last traces leave after about 24 hours. That is the number that decides whether yesterday still counts.

Try it with your drink

Half-life 5.7 h · peak at 45 min · sleep threshold 40 mg

Drink

When you had it

Bedtime

Metabolism

Fast Average Slow

At bedtime

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Below 40 mg at

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Latest for tonight

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The first hour

Caffeine is absorbed almost completely from the gut, which is unusual and part of why the effect is so reliable. Blood levels climb steeply and reach their maximum around 45 minutes in.

Food slows the climb without changing the total. Coffee on an empty stomach peaks sooner and harder, which is what people mean when they say the morning coffee hits differently. A carbonated energy drink on an empty stomach is the fastest common combination, peaking closer to 30 minutes.

What you feel is not caffeine doing something to you directly. Adenosine builds up in the brain through the day and makes you sleepy, and caffeine occupies the receptors adenosine would otherwise use. The adenosine keeps accumulating behind the blockade. When the caffeine clears, all of it lands at once, which is the crash.

The 5.7 hour half-life

Elimination follows first-order kinetics, meaning a constant fraction leaves per unit time rather than a constant amount. Starting from a 96 mg mug of filter coffee:

This is why a two o'clock coffee is a genuine question and a nine o'clock coffee is not. The curve is shallow in the region where you care about it.

Half-life is not "how long it lasts". After one half-life you still have half a dose in you, which for a large drink is more than a small drink was to begin with.

Why yesterday still counts

At 5.7 hours, a dose taken at 8 am is down to roughly 5 percent by 8 am the next day. For a single cup that is negligible. For a heavy day it is not.

Someone drinking 500 mg across a day starts the next morning with 25 to 40 mg already circulating, and never fully returns to zero. That baseline is part of why the first coffee of the day stops feeling like it does anything: you are not lifting yourself from nothing, you are topping up.

What changes the 5.7 hours

That typical value hides a wide range. Healthy adults run from about 1.5 to 9.5 hours, and specific circumstances move it further:

• Oral contraceptives: roughly double, to 10 hours or more.
• Pregnancy: up to 15 hours by the third trimester.
• Smoking: down to about 3 hours. Quitting reverses this within days.
• Liver disease: substantially longer, sometimes several times.
• Regular heavy use: barely changes the half-life at all.

That last one matters. Tolerance is real, but it happens in the brain, not the liver. Your receptors adapt so you feel less, while the caffeine clears at exactly the same rate and goes on interfering with your sleep architecture. People who say caffeine does not affect them are usually describing the first half of that sentence.

What it means in practice

For sleep, the useful figure is not when caffeine is gone but when it drops below about 40 mg, the level where it stops measurably delaying sleep onset. For a normal mug that is about seven and a half hours.

For a crash, expect it three to six hours in, as the level falls through the range where the blockade breaks down and the accumulated adenosine arrives. A second cup postpones it rather than preventing it.

For a nap, the 45 minute peak is the whole trick behind the coffee nap: drink, sleep 20 minutes, and wake as the caffeine arrives on top of naturally cleared adenosine.

For quitting, the withdrawal headache starts 12 to 24 hours after the last dose, which is one to two half-lives past the point where the receptors expected more.