How to Calculate Your Caffeine Cut-Off Time
“How late is too late for coffee” has a real answer, and it is arithmetic rather than opinion. Caffeine is eliminated roughly exponentially, which means the useful number is its half-life: the time it takes for half of a dose to clear.
Population averages put caffeine’s half-life at about five hours, with a normal range of roughly three to seven, and outliers on both sides. That range is why one person drinks espresso at 9 p.m. and sleeps fine while another is awake at 2 a.m. after a 4 p.m. green tea.
The formula
The amount remaining at bedtime is:
remaining = dose × 0.5 ^ (hours elapsed ÷ half-life)
For a decision you can act on, invert it: choose an acceptable residual, then solve for the hours before bed.
hours before bed = half-life × log2(dose ÷ acceptable residual)
If you want no more than 25 mg of a 100 mg dose still circulating — a quarter of the original — and your half-life is five hours, you need 5 × log2(4) = 10 hours. For a 200 mg coffee, that means the last cup is at lunchtime for a 10 p.m. bedtime.
The lookup table
Assuming a five-hour half-life, 11 p.m. bedtime, and a target residual under 25 mg:
| Drink | Caffeine | Last sip by |
|---|---|---|
| Filter coffee, 12 oz | ~180 mg | 12:30 pm |
| Espresso, double | ~125 mg | 2:00 pm |
| Cold brew, 12 oz | ~250 mg | 10:30 am |
| Black tea, 8 oz | ~47 mg | 6:00 pm |
| Green tea, 8 oz | ~28 mg | 7:30 pm |
| Energy drink, 16 oz | ~160 mg | 1:00 pm |
| Cola, 12 oz | ~34 mg | 7:00 pm |
| Decaf coffee, 8 oz | ~5 mg | Any time |
Two things this table does not capture, and both matter. First, dose scales faster than most people think — a large cold brew can carry more caffeine than two espressos. Second, absorption takes 45–60 minutes, so a cup at 3 p.m. peaks around 4 p.m., not at the first sip.
What changes your half-life
Genetics. CYP1A2 and AHR variants shift clearance substantially between individuals. There is no practical home test, but your own history is a decent proxy: if coffee at 4 p.m. costs you sleep, you are a slow metaboliser regardless of what the average says.
Oral contraceptives and pregnancy. Clearance slows markedly — the half-life roughly doubles in late pregnancy, and oestrogen-containing contraceptives slow it too. The commonly quoted 200 mg daily limit in pregnancy reflects this.
Smoking accelerates clearance, which is part of why smokers report stronger withdrawal.
Liver function and some drugs — fluvoxamine, ciprofloxacin and others inhibit CYP1A2 and can double the half-life.
Sleep debt and timing. Caffeine does not remove the need for sleep; it blocks adenosine receptors. Under sleep restriction, the same dose does less for alertness while still disrupting the following night’s sleep architecture.
Why the residual matters more than the dose
Most of the “coffee does not affect me” reports come from people who fall asleep fine and then wonder why their deep sleep is short. Caffeine at bedtime does two things: it lengthens the time to fall asleep, and — at doses that do not obviously delay sleep onset — it reduces slow-wave sleep and increases arousals.
That second effect is the one that hides. You can fall asleep in ten minutes and still wake at 3 a.m. because your sleep architecture was disturbed. Our article on why you wake up at 3 AM covers that pattern, and how caffeine affects sleep covers the mechanism in more depth.
A practical three-week experiment
If you want your own number rather than the population average:
1. For one week, keep your usual intake and log bedtime, sleep onset, and any 3 a.m. waking 2. For week two, move your last caffeine to 12:00 and change nothing else 3. For week three, move it to 14:00
Compare sleep onset latency and the number of night wakings. Most people find a clear break point. That break point is your cut-off, and it is more reliable than any table — including this one.
The bedtime calculator on our tools page handles the sleep-cycle side of the same question: given a wake time, which bedtimes land at the end of a 90-minute cycle.
The safety ceiling
The FDA cites 400 mg per day as a level not generally associated with harmful effects in healthy adults — roughly four 8 oz cups of filter coffee. That is a ceiling, not a target, and it does not apply in pregnancy, to people with arrhythmias, or to children and adolescents, who should keep intake much lower.
Caffeine is also in places people do not count: pre-workout powders, some headache tablets, chocolate, and “decaf” that is not fully decaffeinated. If you are trying to cut down, read labels for a week before deciding it is not working.
FAQ
Is caffeine after 2 p.m. always a problem?
No. It depends on your half-life and your bedtime. Someone with a three-hour half-life clearing a green tea at 5 p.m. is fine; someone with a seven-hour half-life drinking a large cold brew at noon may not be.
Does decaf really have no caffeine?
It has a small amount — typically 2–15 mg per cup, depending on the process. That is not enough to matter for most people, but it is not zero.
Can I “flush” caffeine by drinking water?
No. Clearance is hepatic, and water does not speed it up. Hydration helps with the headache of withdrawal, which is a different problem.
Does caffeine lose its effect with tolerance?
The alerting effect attenuates with regular use, but the sleep-disrupting effect persists. That asymmetry is why heavy consumers often stop noticing the benefit while still paying the cost.
This is general information, not medical advice. If you have a heart condition, are pregnant, or take medication that interacts with caffeine, talk to a clinician — see our medical disclaimer.