How much caffeine is safe per day?

For most healthy adults, up to about 400 mg a day is generally considered safe — roughly four 240 ml cups of brewed coffee. During pregnancy, guidance is lower, around 200 mg. Above these levels, side effects like anxiety, a racing heart, and insomnia become more likely, though individual tolerance varies a lot.

What is caffeine's half-life?

Caffeine's half-life — the time for your body to clear half of it — is roughly 4 to 6 hours in a typical adult. So if you have 200 mg at 3pm, around 100 mg can still be in your system at 8–9pm. It takes several half-lives to clear almost completely, which is why afternoon caffeine can disrupt sleep.

Why does afternoon coffee affect sleep?

Because of that long half-life. Caffeine works by blocking adenosine, the molecule that builds sleep pressure through the day. A late dose keeps blocking it into the evening, making it harder to fall asleep and reducing deep sleep — often without you realising, since you may still fall asleep but sleep less restfully.

How much caffeine is in common drinks?

A 240 ml cup of brewed coffee has about 95 mg; an espresso shot about 63 mg; instant coffee around 62 mg; black tea about 47 mg; green tea about 28 mg; a cola about 34 mg; and an energy drink roughly 80 mg per 250 ml. Café servings are often larger, so a big latte can carry 150 mg or more.

Why do some people handle caffeine better than others?

Largely genetics. People vary in how quickly their liver metabolises caffeine, so 'fast metabolisers' clear it rapidly and tolerate more, while 'slow metabolisers' feel effects longer and more strongly. Pregnancy, some medications, and liver function also change how fast you process it, which is why a single late coffee wrecks some people's sleep but not others'.

How Much Caffeine Is Too Much? Half-Life, Limits, and Your Sleep

The short version

~400 mg/day is the general safe ceiling for healthy adults (~200 mg in pregnancy).
Caffeine's half-life is 4–6 hours — half an afternoon dose can linger to bedtime.
It blocks adenosine, the molecule that builds sleep pressure — hence disrupted sleep.
Sensitivity is largely genetic, so the same coffee hits people very differently.
To tally your daily intake against the limit, use the Caffeine Calculator.

The drug we don't think of as a drug

Caffeine is the world's most widely used psychoactive substance, and we treat it as casually as water. Mostly that's fine — moderate caffeine is safe and even has upsides for alertness and performance. But "moderate" has a number, caffeine lingers far longer than its buzz suggests, and the gap between people's tolerance is enormous. Knowing how it works turns caffeine from something that happens to you into something you can actually manage.

The safe ceiling

For most healthy adults, health authorities put the general safe limit at about 400 mg of caffeine a day — roughly four 240 ml cups of brewed coffee. Stay under that and, for most people, caffeine is benign.

Push past it and the familiar downsides get more likely: jitteriness, anxiety, a racing or pounding heart, stomach upset, and disrupted sleep. The limit is lower for some groups — pregnancy guidance is around 200 mg a day, and people with certain heart conditions, anxiety disorders, or caffeine sensitivity may need less. The Caffeine Calculator adds up your intake across drinks so you can see where you land against the 400 mg mark, because it's easy to underestimate.

Where it adds up

Caffeine hides in more than coffee. Typical amounts:

Brewed coffee (240 ml): ~95 mg
Espresso shot: ~63 mg
Instant coffee: ~62 mg
Black tea: ~47 mg
Green tea: ~28 mg
Cola (355 ml): ~34 mg
Energy drink (250 ml): ~80 mg

The catch is serving size: a large café coffee or a double-shot latte can carry 150 mg or more, and a big energy drink can top 160 mg. A couple of generous coffees plus a tea and a cola can quietly cross 400 mg without a single "big" caffeine hit. This is why tallying actual intake, rather than counting "cups," is worthwhile.

The half-life problem

Here's the part that catches people out. Caffeine has a half-life of roughly 4–6 hours — the time for your body to clear half of a dose. So a 200 mg hit at 3pm leaves about 100 mg still circulating at 8–9pm, and a meaningful amount past midnight. It takes several half-lives to clear almost entirely, which means afternoon caffeine is still working at bedtime.

That slow clearance is the mechanism behind caffeine's most underappreciated cost: sleep. Which brings us to how it works.

Why it steals sleep (even when you fall asleep fine)

Through the day, a molecule called adenosine builds up in your brain, creating "sleep pressure" — the growing urge to sleep. Caffeine works by blocking adenosine's receptors, masking that pressure and making you feel alert. The problem is that a late dose keeps blocking adenosine into the evening.

The insidious part: you may still fall asleep, so you assume the coffee was harmless — but research shows caffeine late in the day reduces deep, restorative sleep and total sleep time even when you don't notice trouble dropping off. The practical rule that follows from the half-life: if caffeine affects your sleep, cut it off by early-to-mid afternoon. Those last few hours of blocking matter more than they feel like they should.

Why your friend can drink espresso at 10pm

If you've ever marvelled at someone downing coffee after dinner and sleeping fine, the answer is mostly genetics. People vary widely in how fast their liver enzymes metabolise caffeine. Fast metabolisers clear it quickly and tolerate more; slow metabolisers feel it longer and harder, and a single late coffee can wreck their night. Pregnancy, some medications, and liver function also shift the rate.

This genetic lottery is why there's no universal caffeine rule beyond the broad safe ceiling — you have to learn your own response. If you sleep badly and drink caffeine after lunch, you may simply be a slow metaboliser paying a price you hadn't connected to the coffee.

Tracking your intake

Add up your drinks in the Caffeine Calculator to see your daily total against the safe limit and when it roughly clears. If sleep is the concern, pair it with the Sleep Calculator to time your bedtime around your cycles.

Sources

US Food and Drug Administration (FDA). Spilling the Beans: How Much Caffeine is Too Much.
European Food Safety Authority (EFSA). Scientific Opinion on the Safety of Caffeine.
Drake, C. et al. "Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before Going to Bed," Journal of Clinical Sleep Medicine (2013).

This article is for general informational and educational purposes only and is not medical advice. Caffeine tolerance varies; those who are pregnant, have heart conditions, or take medication should consult a doctor about their caffeine intake.

Written by

Tufail Ahmed

Computer Scientist

Tufail builds CalcPool's calculators and writes the research behind them, turning documented formulas and primary sources into tools and explainers you can actually use.

Reviewed by Khizar Nadim · Technical Reviewer

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