How much each coffee really contains, why your friend can drink it at night and you cannot, and what the health studies actually say — which is less than is usually claimed.
What it does in the body
Caffeine doesn't give you energy. What it does is block adenosine receptors — adenosine being the molecule that accumulates through the day and tells the brain you're tired.
The adenosine is still there and still accumulating; you simply don't perceive it. Which is why the crash arrives once the caffeine clears: you get back, all at once, the tiredness that had been piling up meanwhile.
How much each coffee contains
Figures vary with the bean and the brew, but as an order of magnitude:
| Drink | Approx. caffeine |
|---|---|
| Single espresso (7-9 g) | 60-80 mg |
| Double espresso (18 g) | 110-150 mg |
| Filter coffee (300 ml) | 150-200 mg |
| Capsule | 55-80 mg |
| Moka pot (cup) | 80-120 mg |
| Black tea | 40-70 mg |
| Decaf | 2-5 mg |
Note something that surprises nearly everyone: a large filter coffee contains more caffeine than an espresso. Espresso is far more concentrated per millilitre, but it's served in 40 ml and uses less coffee overall. Caffeine is determined by the grams of coffee used, not by perceived intensity.
The limits, according to EFSA
The European Food Safety Authority sets three reference points for healthy adults:
- 400 mg a day — about 3-4 coffees, with no expected adverse effects.
- 200 mg in one sitting — the recommended maximum in a single dose.
- 200 mg a day during pregnancy and breastfeeding.
These are population-level references. Individual tolerance varies enormously, and there's a specific reason for that.
Why it affects everyone differently
Caffeine is metabolised in the liver by an enzyme called CYP1A2, and which genetic variant you carry determines how fast that happens.
Fast metabolisers clear it within a few hours and can have coffee after lunch without consequence. Slow metabolisers carry it far longer, and a coffee at five in the afternoon affects their sleep in the small hours.
It isn't a matter of toughness or habit: it's a genuine biochemical difference. If coffee keeps you awake and doesn't affect someone else, neither of you is exaggerating.
Half-life and sleep
Caffeine's half-life is around 5 hours: after that you still have half of it in your system. Run the numbers on a 150 mg coffee at 17:00 — by 22:00 you have 75 mg left, and at 03:00 still around 19.
And here's the important part: even if you manage to fall asleep, caffeine reduces deep sleep, which is the restorative phase. You can sleep your full hours and wake up unrested.
The practical recommendation is to stop 8 to 10 hours before bed. For someone going to sleep at eleven, that means the last coffee in early afternoon.
Tolerance and withdrawal
With regular consumption the body produces more adenosine receptors to compensate for the blockade. That's the mechanism of tolerance: you need the same dose just to feel normal, not to feel better.
Stop abruptly and those extra receptors are left unoccupied, producing headache, irritability and mental fog for two to nine days. Tapering over a couple of weeks, you'll barely notice.
What the health studies say
Precision matters here, because this subject gets reported very loosely.
There is a great deal of research associating moderate coffee consumption with lower risk of type 2 diabetes, some liver diseases and certain neurodegenerative conditions. Most of it is observational: it finds correlation, it doesn't demonstrate cause.
That distinction matters. People who drink coffee in moderation may differ from those who don't in many other respects, and those other respects may explain part of the result. The honest reading is that coffee in moderation doesn't appear harmful and probably contributes something, not that it's a medicine.
When to cut back
If you have anxiety or palpitations, if you sleep badly, if you suffer from reflux, or if you're pregnant — in which case the limit is 200 mg daily.
Cutting back doesn't mean giving up the ritual: a well-processed decaf keeps almost all the flavour at 2-5 mg a cup. We explain it in the decaf guide. And if caffeine interests you as a performance aid, that's in coffee and sport. With and without caffeine, in the shop.
