01The Point Where More Becomes Less
One cup is focus. Two is momentum. Somewhere between the third and the bottom of an overfilled travel mug, coffee stops working for you and starts working on you: jaw tight, thoughts skipping, a low irritability that makes even small decisions feel faintly hostile.
This isn't weakness or imagination. Caffeine works by blocking adenosine1 receptors — the chemical signalling that accumulates through the day and nudges you toward tiredness. Block enough of it and you feel alert; overshoot and you raise cortisol2, the stress hormone your body would otherwise reserve for emergencies. The result is a physical state that closely resembles anxiety without the cause: heart slightly too fast, attention slightly too scattered, calm slightly out of reach.
The threshold is genuinely individual. Genetics influence how quickly you metabolise caffeine, and so does sleep debt, stress, whether you've eaten, and your cycle if you have one. What worked fine on a rested Tuesday can wire you thoroughly on a depleted Friday.
A few useful anchors: caffeine's half-life3 in most adults sits between four and six hours, so a 3pm coffee is still partly active at bedtime. The tense, over-caffeinated feeling often arrives before you recognise it as caffeine — it just feels like the day getting on top of you.
The fix is rarely 'no coffee'. It's usually earlier, smaller, and with food — because caffeine on an empty stomach hits faster and steeper. Noticing where your own line sits is genuinely useful information.
Kitchen notes
This article is general guidance, not medical advice. If anxiety or sleep problems are a persistent concern, a GP is the right conversation.