Most people treat coffee like a light switch: you drink it, it turns you on for a few hours, and then it wears off. In reality, caffeine behaves like any drug with a half-life, and that half-life varies from person to person.
What is a Half-Life?#
A half-life is the time it takes your body to clear half of a dose. For caffeine in a healthy adult, it's roughly 4 to 7 hours depending on the person (around 5 on average). Genetics, smoking, pregnancy and some medications (including oral contraceptives) all shift it. So 4 to 7 hours after you finish your coffee, half of the caffeine is still in your bloodstream, blocking adenosine receptors in your brain and making deep sleep lighter.
The 3 PM Espresso Math#
Let's look at the math of a 3:00 PM espresso (which typically contains about 100mg of caffeine), assuming an average 5.5-hour half-life (ARC asks whether yours is closer to 4, 5.5 or 7 hours):
- 3:00 PM: 100mg active
- 8:30 PM: 50mg still active
- 2:00 AM: 25mg still active
If you go to bed at 11:00 PM, you still have about 37 mg active. That's under the 50 mg line ARC uses, so one espresso at 3 PM usually fits an 11 PM bedtime. A 200 mg coffee at 3 PM wouldn't: you'd still carry about 73 mg at 11 PM. And larger doses late in the day add up fast: in one well-known study, 400 mg taken even six hours before bed cut total sleep by more than an hour (Drake et al. 2013).
Calculate Your Bedtime Decay: Use our free Caffeine Half-Life Calculator to view your exact hourly decay graph and determine your personalized sleep-safe cutoff time.
How to Stop the Cycle#
There's no universal cutoff. For one average cup and an average metaboliser, caffeine takes roughly five hours to fall below 50 mg; a bigger cup, a slower metabolism, or an earlier coffee still in your system pushes that earlier. Doing this math in your head every time you have a cup of tea or an energy drink is close to impossible. (If you just want the practical version, see when to stop drinking coffee.)
This is the flagship feature of ARC. The app shows a live caffeine decay curve: you log your drink, and ARC shows how many milligrams are still active in your system. It works out your last safe coffee: the latest you can have your next realistic drink and still be under 50 mg at bedtime, based on your bedtime, your sensitivity and every drink logged today. If no cup fits, it tells you that too.
For a deeper walkthrough of dose, timing and sensitivity, read the caffeine half-life optimization guide. If coffee has stopped working for you altogether, that's a tolerance question: see why coffee makes you tired.
Scientific References#
- Drake, C., et al. (2013). Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 9(11), 1195-1200.
- Clark, I., & Landolt, H. P. (2017). Coffee, caffeine, and sleep: A systematic review of epidemiological studies and randomized controlled trials. Sleep Medicine Reviews, 31, 70-78.