Caffeine science
How caffeine works, how long it lasts, and what makes its effect differ from one person to the next. Every article on the mechanism.
Caffeine works by blocking adenosine, the molecule that builds up through the day and makes you feel tired. It does not add energy — it hides the signal that you are running low, which is why the tiredness is still waiting for you when the effect fades.
The number that decides your evening is the half-life: roughly five hours in an average adult, so half of the coffee you drank at 4 p.m. is still circulating at 9 p.m. That single figure explains most of what the articles below cover. Start with what caffeine is, then with how long it lasts.
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How long does caffeine stay in your system?
Caffeine has a half-life of around five hours on average — but it ranges from two to ten. Here is what that means, and what makes your number different.
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How much caffeine per day is safe?
General daily caffeine reference points, why your personal limit differs, and how to stay under it while still enjoying your coffee.
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Caffeine in coffee, tea & energy drinks: the chart
A clear, sourced reference for caffeine content per drink — espresso, brewed coffee, tea, energy drinks, and soft drinks, in mg per serving.
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How long does caffeine take to kick in?
Caffeine starts working in ~15–20 minutes and peaks around 30–60. Time your coffee ahead of when you need it — and what changes the speed.
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How a caffeine calculator works
What a caffeine calculator actually computes, what it needs to be accurate, and where every one of them stops — including ours.
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Fast or slow caffeine metabolism: the CYP1A2 gene
Why some people can drink espresso at night and others can't — the science of caffeine metabolism, the CYP1A2 gene, and what it means for you.
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How to find your personal caffeine limit
You can't look up your caffeine limit — you find it. A simple, no-DNA-test method to discover your own line in milligrams and by time of day.
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What is caffeine and how does it work?
Caffeine is a natural stimulant that blocks adenosine, the chemical that makes you sleepy — so it masks tiredness rather than adding energy. The full explainer.
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Does caffeine actually improve focus?
Yes — caffeine sharpens alertness, vigilance and reaction time, most when you're tired. But there's a sweet spot: too much wrecks focus. How to use it well.
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Caffeine and cortisol: does coffee spike stress?
Yes — caffeine can modestly raise cortisol, but the rise is small and regular drinkers build tolerance. The morning-timing myth, and whether it matters.
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Does caffeine boost metabolism or burn fat?
Yes — caffeine gives metabolism a small, temporary boost and nudges up fat burning. But it's modest, fades with tolerance, and isn't a weight-loss shortcut.
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ANSES on caffeine: what France's agency says
The French food-safety agency sets no maximum dose. It measures what people actually drink, and rules on two specific pairings. Here is the detail.
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Why caffeine guidelines differ between countries
Everyone agrees on 400 mg a day. The divergences are elsewhere — and the trap is that a US 'cup' is 355 ml, not an espresso.