Coffee kicks us into gear in the morning, saves us during the day, and is often a pleasant ritual. But what actually happens in the body when we have an espresso? And why does coffee sometimes "not work" on us – or conversely, keeps us awake until two in the morning?
The brain creates a substance during the day called adenosine. The more there is, the more tired we feel – it's a natural biological signal that the body needs to rest.
Caffeine is like an actor in this story who pretends to be adenosine. It sits in its place in the brain (on the so-called adenosine receptors), but does not activate them. By doing this, it actually blocks the fatigue signal – and we feel fresh and alert.
The problem is that the adenosine doesn't disappear in the meantime. It keeps accumulating. And when the caffeine wears off after a few hours (its half-life is approx. 5–7 hours), all that accumulated adenosine "crashes" down on us – and sudden fatigue sets in. That's why we often feel exhausted later after coffee, not right away.
In the book Why We Sleep , neuroscientist Matthew Walker points out that caffeine has a very long reach. Even if you have your last coffee at 15:00, at 22:00 there can still be half of the dose active in your body.
This doesn't mean you won't fall asleep – but your sleep will be shallower and deep sleep will be disrupted, which is crucial for regeneration, memory, and immunity. Often we don't even realize it – we just wake up more tired in the morning, so we have another coffee… and the cycle continues.
