The short answer
It has a half life of around five hours, so a coffee at four in the afternoon still has half its caffeine working at nine. Stopping by early afternoon is the useful change.
- Half life of roughly five to six hours in most adults, and longer in some.
- A 3pm coffee means a meaningful dose still circulating at bedtime.
- Set a cut-off in the early afternoon and keep it consistent.
- Watch the hidden sources: energy drinks, cola, strong tea, some painkillers and dark chocolate.
- Caffeine damages sleep quality even where it does not delay sleep onset, which is why people who "can sleep fine after coffee" still wake unrefreshed.
What will ruin it
Stopping abruptly after a heavy habit causes headaches and irritability for a few days. Taper rather than quitting overnight.
Why it works
Caffeine blocks adenosine, the molecule that accumulates through the day and creates the feeling of sleepiness. It does not remove the tiredness, it hides the signal, and the pressure is still there underneath.
This is general information, not medical advice, and it does not replace a GP, a pharmacist or NHS 111. Call 999 in an emergency. Full disclaimer.