"I cannot quit coffee, but I want to sleep." That double wish belongs to nearly every Korean office worker. Good news: both are possible — if you get the timing right. This article condenses the pharmacology of caffeine and shows you how to compute your own personal cutoff.
What a 5–6 hour half-life means
Caffeine's average half-life is 5–6 hours — the time it takes your body to break down half of what you drank. So:
- 1 PM — 100 mg consumed
- 7 PM — 50 mg remaining
- 1 AM — 25 mg remaining
Even at lights-out, 25 mg — about a cup of green tea — is still working. That alone is enough to measurably reduce the proportion of deep-sleep stages.
Why some people drink coffee at dinner and sleep fine
Caffeine is broken down by the liver enzyme CYP1A2, and genetic variation makes one person's metabolism more than twice as fast as another's.
Fast metabolizers have a 3–4 hour half-life. Evening coffee barely registers. About 40% of the population.
Slow metabolizers have a 7–9 hour half-life. A single after-lunch cup can delay sleep onset by an hour. About 10% of the population.
Pregnancy, oral contraceptives, and certain antibiotics slow caffeine metabolism further. Smoking speeds it up — which is why ex-smokers often find caffeine suddenly stronger after they quit.
Caffeine content by drink
| Drink | Serving | Caffeine |
|---|---|---|
| Starbucks Tall Americano | 355 ml | ~150 mg |
| Convenience-store can coffee | 200 ml | ~70 mg |
| Bottled latte | 250 ml | ~80 mg |
| Black tea (1 bag) | 200 ml | ~50 mg |
| Green tea (1 bag) | 200 ml | ~30 mg |
| Cola (can) | 355 ml | ~35 mg |
| Dark chocolate (50 g) | — | ~30 mg |
| Energy drink (Red Bull 250 ml) | 250 ml | ~80 mg |
Your personal cutoff: a quick formula
Subtract 8 hours from your target bedtime.
- Want to sleep at 11 PM? Cutoff at 3 PM.
- 10 PM? Cutoff at 2 PM.
- 9 PM? Cutoff at 1 PM.
If you suspect you're a slow metabolizer, subtract 10 hours instead. 10 PM bedtime = noon cutoff. Switch the after-lunch cup to decaf.
A one-week experiment
- Mon–Fri, log the time and amount of your last caffeine.
- Roughly note how long it took to fall asleep.
- Look at the weekend — were the late-cutoff nights also the slow-onset nights?
- The following week, push your cutoff one hour earlier and run the same measurement.
For most people, "cutoff one hour earlier" is the single biggest variable that shortens sleep onset — typically by 15 to 25 minutes. Earlier cutoff leads to deeper sleep; deeper sleep leads to less caffeine craving the next day. The virtuous loop starts there.
One caution — taper, do not quit cold
Long-term coffee drinkers who quit abruptly often face headaches, fatigue, and low mood within 24–48 hours. The withdrawal fades in 1–2 weeks, but if those weeks feel impossible, taper instead — drop 50 mg per week.
You do not need to give up the pleasure of coffee. You only need to move it. Eleven AM instead of 4 PM. Same pleasure, sleep returned.
