How does caffeine keep you awake?
Caffeine blocks the receptors in your brain that detect adenosine, a substance that builds up during the day and makes you feel sleepy. Your tiredness is still there, your brain just can’t feel it for a while.
Caffeine travels to your brain
Caffeine is absorbed through your stomach and gut into your blood. It reaches your brain within about 15 to 45 minutes, which is why that first sip doesn’t work instantly.
Share this stepYour brain keeps score of tiredness
While you’re awake, your brain cells release a substance called adenosine. It builds up through the day and docks onto special receptors, and the more that docks, the sleepier you feel.
Share this stepCaffeine takes the parking spots
Caffeine is shaped enough like adenosine to fit into the same receptors, but it doesn’t trigger them. It sits there like a key that fits but won’t turn, so the sleepiness signal can’t get through.
Share this stepCaffeine hides your tiredness, it doesn’t remove it.
Caffeineblocked receptorsawake
When the caffeine wears off, all the adenosine that built up meanwhile floods the receptors at once. That’s the afternoon crash.
Common follow-up questions
How long does caffeine last?
Your body clears about half the caffeine in roughly 5 hours, though it varies a lot between people. A coffee in the afternoon can still affect your sleep at night.
Why does coffee stop working if I drink it every day?
Your brain adapts by growing more adenosine receptors, so you need more caffeine for the same effect. A short break resets it.
How much caffeine is too much?
For most healthy adults, up to about 400 mg a day, roughly four cups of brewed coffee, is considered safe. Pregnant people and teenagers should have less.
Does decaf have any caffeine?
A little. A cup of decaf usually contains a few milligrams, compared with about 80 to 100 mg in regular coffee.
