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?
During the day, the brain produces a substance called adenosine. The more there is, the more tired we feel – it is a natural biological signal that the body needs to rest.
Caffeine in this story acts like an actor pretending to be adenosine. It binds to its place in the brain (the so-called adenosine receptors), but does not activate them. By doing so, it actually blocks the fatigue signal – and we feel refreshed and alert.
The problem is that adenosine doesn't disappear in the meantime. It keeps accumulating. And when caffeine wears off after a few hours (its half-life is about 5–7 hours), all that accumulated adenosine comes crashing down on us – and sudden fatigue sets in. That's why we often feel exhausted later after having coffee, rather than 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 half of the dose may still be active in your body.
This doesn't mean you won't fall asleep – but sleep will be lighter and deep sleep will be disrupted, which is crucial for recovery, memory, and immunity. Often we don't even realize it – we just wake up more tired in the morning, so we grab another cup of coffee... and the cycle continues.
