Caffeine's half-life is about five hours in a typical healthy adult: five hours after a coffee, half its caffeine is still in your body, and a quarter after ten. That's why an afternoon coffee can still be working at bedtime. For most people, the last cup should come at least six to eight hours before bed.
What's left of one coffee
A typical 95 mg cup of brewed coffee, with a five-hour half-life:
| After | Caffeine left |
|---|---|
| Drinking it | 95 mg |
| 2 hours | 72 mg |
| 5 hours | 48 mg |
| 8 hours | 31 mg |
| 10 hours | 24 mg |
| 15 hours | 12 mg |
A coffee at 4:00 PM leaves about 36 mg in you at 11:00 PM, roughly a cup of green tea's worth, just as you try to sleep.
Your half-life may be different
Five hours is an average. In healthy adults it ranges from around two hours to eight or more, depending on genes, age and more:
- Faster: smoking roughly halves it.
- Slower: pregnancy (much slower, especially late on), oral contraceptives, liver disease, and some medications, including certain antidepressants and antibiotics.
- Genes: variations in the liver enzyme that breaks caffeine down (CYP1A2) make some people fast metabolisers and others slow.
How caffeine affects sleep
Caffeine works by blocking adenosine, the chemical that builds up while you're awake and makes you feel sleepy. Block it at bedtime and you fall asleep later and sleep more lightly. In a 2013 study by Christopher Drake and colleagues, 400 mg of caffeine taken six hours before bed still cut sleep by more than an hour, and the participants mostly didn't notice. Caffeine can steal sleep without you feeling it.
When to have your last coffee
- Going to bed at 11 PM? Last coffee by about 3 PM; earlier if you're sensitive or it's a strong one.
- Big cups count double. A large coffee-shop coffee can hold two or three times a home cup.
- Don't forget the hidden ones: tea, cola, energy drinks, pre-workout, dark chocolate, and some painkillers.
The caffeine calculator adds up your actual day, drink by drink, and shows what's left at your bedtime.
Caffeine for focus, timed well
- Have it with your hardest work, usually the morning. A moderate dose improves alertness and reaction time.
- Try a coffee nap after lunch: coffee, then a 15 to 20-minute nap, waking just as it kicks in. See how long a power nap should be.
- Don't use it to cover a short night every day. Caffeine hides sleepiness; it doesn't repay the debt, and late doses make the next night shorter too.
Health authorities such as the US FDA say up to 400 mg a day is not generally associated with negative effects in healthy adults; pregnant people are usually advised to stay under 200 mg. Ask your doctor if you're unsure.
Tolerance and withdrawal
If you drink caffeine every day, your body adapts within days: you feel less of a boost, and part of the morning coffee's effect is simply undoing the overnight dip of a mild withdrawal. Stop suddenly and withdrawal symptoms, such as headache, tiredness, low mood and poor concentration, typically start 12 to 24 hours after the last dose, peak after a day or two, and can last up to about nine days, according to a review by Laura Juliano and Roland Griffiths. Cutting down gradually, by a quarter every few days, avoids most of it.
One practical upshot: if you want caffeine to work for a big day, such as an exam, a long drive or the day after a red-eye, a lower daily habit makes the occasional extra cup far more effective.
Coffee, then focus
The hour after your morning coffee is prime focus time. In Flight, book it as a real flight and work until you land.