Coffee gives us a kick 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 doesn’t coffee sometimes “catch” us – or, conversely, keep us up until 2 a.m.?
Adenosine: the silent hero (or villain) of fatigue
The brain makes a substance called adenosine during the day . The more of it there is, the more tired we feel – it’s a natural biological signal that the body needs to rest.
Caffeine is like an actor pretending to be adenosine in this story. It sits on its place in the brain (the so-called adenosine receptors) but does not activate them. This actually blocks the fatigue signal – and we feel refreshed and awake.
The problem is that adenosine doesn’t go away in the meantime. It keeps on accumulating. And when the caffeine wears off after a few hours (its half-life is about 5-7 hours), all that accumulated adenosine “crashes” down on us – and sudden fatigue sets in. That’s why after coffee we often feel exhausted later, not right away.
Why coffee disturbs sleep (even if you drink it “only” in the afternoon)
In Why We Sleep, neuroscientist Matthew Walker points out that caffeine has a very long reach. Even if you have your last coffee at 3pm, by 10pm half the dose may still be active in your body.
This doesn’t mean you won’t fall asleep – but sleep will be shallower and deep sleep, which is crucial for recovery, memory and immunity, will be disrupted. Often we don’t even realise it – we just wake up more tired in the morning, so we have another coffee… and the cycle continues.
Tips for conscious coffee drinking
- 🕒 Time your caffeine: ideally have your last coffee 6-8 hours before bedtime.
- ☕ Choose a smaller dose: Sometimes a single espresso is enough instead of a double.
- 🌱 Try decaffeinated coffee in the afternoon – you’ll enjoy the taste without affecting your sleep.
- 🧠 Feel the effect: Listen to your body – does coffee really make you feel better, or is it just a habit?
