When should you stop drinking coffee?
The FDA gives caffeine a half-life of four to six hours in a healthy adult. Five is the usual working figure, and it means the decay is slower than people expect: caffeine does not wear off, it halves, and halving from a large enough starting dose still leaves a meaningful amount at bedtime.
What is left, hour by hour
| Hours since the cup | Share remaining | From a 95 mg coffee |
|---|---|---|
| 0 | 100% | 95 mg |
| 2 | 76% | 72 mg |
| 4 | 57% | 55 mg |
| 5 | 50% | 48 mg |
| 6 | 44% | 41 mg |
| 8 | 33% | 31 mg |
| 10 | 25% | 24 mg |
| 12 | 19% | 18 mg |
| 15 | 13% | 12 mg |
| 20 | 6% | 6 mg |
The half-life is not five hours for everyone
- The CYP1A2 enzyme does most of the clearing, and its activity varies several-fold between people on genetic grounds. Fast metabolisers really can drink coffee at 9 pm.
- Combined oral contraceptives roughly double the half-life.
- Pregnancy extends it substantially, up to around fifteen hours in the third trimester.
- Smoking roughly halves it, which is why people who quit often find their usual coffee suddenly too strong.
- Liver impairment extends it, sometimes greatly.
What caffeine does to sleep specifically
Caffeine blocks adenosine receptors. Adenosine accumulates through the day and is the chemical basis of sleep pressure, so blocking it does not add energy — it hides tiredness that is still there and still accumulating. The debt comes due when the caffeine clears, which is the mechanism behind the afternoon crash.
The measurable effect on sleep is mostly on depth rather than on falling asleep. People who insist coffee does not stop them dropping off are usually right and are still losing slow-wave sleep, which is the part they would miss.
Cut-off times by bedtime
| Bed at | Last coffee for a quarter left | For an eighth left |
|---|---|---|
| 9:00 pm | 11:00 am | 6:00 am |
| 10:00 pm | 12:00 pm | 7:00 am |
| 11:00 pm | 1:00 pm | 8:00 am |
| 12:00 am | 2:00 pm | 9:00 am |
| 1:00 am | 3:00 pm | 10:00 am |
Common questions
- How long does caffeine stay in your system?
- The half-life is about five hours in a healthy adult, so a quarter is left after ten hours and an eighth after fifteen. Full clearance takes well over a day.
- What time should I stop drinking coffee?
- Eight to ten hours before bed for most people — around 2 pm for an 11 pm bedtime. That leaves a quarter to an eighth of the dose, which is where most people stop noticing an effect.
- Why does coffee not stop me falling asleep?
- Caffeine affects sleep depth more reliably than sleep onset. You can fall asleep normally and still lose slow-wave sleep, which is the part that matters most for feeling rested.
- Does caffeine affect everyone the same way?
- No. Clearance depends mostly on the CYP1A2 enzyme, whose activity varies several-fold between people. Oral contraceptives roughly double the half-life, pregnancy extends it much further, and smoking roughly halves it.
Related pages
- What a sleep cycle actually is Stages, and why 90 minutes is an average
- Sleep timing Work a bedtime back from your alarm
Sources
- US Food and Drug Administration — FDA (Public domain (US Government))