Caffeine calculator: how much is still in your body at bedtime

Caffeine doesn't disappear when you stop feeling it

The half-life of caffeine in a healthy adult is about five hours. This means that five hours after your coffee, half of the amount is still circulating. After ten hours, a quarter is left. A 95 mg filter coffee drunk at 16:00 leaves around 36 mg in your body at 23:00, roughly as much as a green tea drunk in bed.

You no longer feel it because the subjective effect drops much faster than the concentration. But caffeine doesn't work by giving you energy: it blocks adenosine receptors. Adenosine is the molecule that builds up in the brain over the day and signals that it is time to sleep. While the receptors are blocked, the signal is there, but it can't be heard.

How long it takes for caffeine to leave your body

In practice, for a 95 mg filter coffee drunk at lunchtime, the maths looks like this:

• After 5 hours: about 48 mg
• After 10 hours: about 24 mg
• After 15 hours: about 12 mg
• After 24 hours: traces

That is why “no more coffee in the afternoon” works for some people and changes nothing for others: what matters is the exact time, the amount and how quickly you metabolise it.

How much caffeine common drinks contain

We used average values, because the content itself varies a lot depending on the beans, the roast and how strong you pour it.

• Espresso, 30 ml: about 63 mg
• Filter coffee, 250 ml: about 95 mg
• Cappuccino or latte: about 80 mg
• Energy drink, 250 ml: about 80 mg
• Matcha, 2 g: about 70 mg
• Black tea, 250 ml: about 47 mg
• Green tea, 250 ml: about 28 mg
• Cola, 330 ml: about 33 mg

The 50 milligram threshold

There is no magic number below which caffeine stops mattering, but around 50 mg at bedtime is the threshold below which most people no longer report trouble falling asleep. That is why the calculator also gives you a cut-off time: the point after which your last dose no longer has time to drop below the threshold before bed.

How much caffeine a day is safe

The European Food Safety Authority has concluded that, for healthy adults, an intake of up to 400 mg a day and single doses of up to 200 mg do not raise safety concerns. During pregnancy, the recommended limit drops to 200 mg a day. Four hundred milligrams is roughly four filter coffees, but if they all come in the afternoon, the bedtime calculation looks completely different than if they come in the morning.

Why some people sleep after an espresso and others don't

The difference is largely genetic. The enzyme that metabolises caffeine, CYP1A2, works faster or slower depending on a fairly common genetic variant; between fast and slow metabolisers, the difference in half-life can be several hours.

On top of that comes context: smoking nearly doubles the speed of metabolism, while pregnancy and oral contraceptives slow it down considerably. Some medicines have the same effect. In other words, “coffee doesn't do anything to me” can be perfectly true for you and completely false for the person next to you.

What to do if the number comes out high

The simplest thing is not to drink less coffee, but to drink it earlier. The same amount, moved three hours earlier, can mean half the caffeine left at bedtime.

The second option is to swap the last drink of the day, not the first. A green tea instead of the four o'clock coffee cuts around 70 mg from the total. And if you want to cut down altogether, do it gradually: stopping abruptly after high intake brings headaches and irritability for a few days, which makes the break harder to get through than it needs to be.

The calculator's limits

The model used is the simplest possible: a single elimination phase with a five-hour half-life. Reality involves absorption, distribution and large individual variation. Treat the result as an order of magnitude, useful for seeing that the afternoon espresso really does matter, not as a measurement.