“Stop at 2 p.m.” is a slogan, not a calculation. Your real cut-off depends on the dose, your bedtime, and a half-life measured across a four-fold range between people.
The short answer
Bedtime minus 8.8 hours, for one normal coffee.
A 2023 meta-analysis in Sleep Medicine Reviews pooled the trials and worked the timing backwards: a 107 mg coffee at least 8.8 hours before bed, a 217.5 mg pre-workout at least 13.2 hours. Pooled, caffeine cut total sleep time by 45.3 minutes and sleep efficiency by 7.0%. Against it: the half-life underneath has been recorded from 2.3 to 9.9 hours across individuals. A starting line, not a law.
Key takeaways
- The quarter-life is the number that matters at bedtime. On a five-hour half-life, 25% of the dose is still circulating ten hours later.
- CYP1A2 does more than 90% of caffeine metabolism, and it’s what varies. Smoking almost doubles clearance; oral contraceptives push half-life from 6.2 to 10.7 hours.
- In the third trimester of pregnancy it stretches to 11.5–18 hours against a habitual 2.5–5.
- Caffeine adds nothing. It blocks adenosine receptors, hiding sleep pressure that keeps building behind the block.
- The FDA’s ceiling is 400 mg a day; one 12 fl oz brewed coffee is 113–247 mg.
Your cut-off, by bedtime
Take the meta-analysis figures and subtract.
| Bedtime | Last ordinary coffee (107 mg) | Last big dose (217.5 mg pre-workout) |
|---|---|---|
| 10:00 p.m. | 1:12 p.m. | 8:48 a.m. |
| 10:30 p.m. | 1:42 p.m. | 9:18 a.m. |
| 11:00 p.m. | 2:12 p.m. | 9:48 a.m. |
| 11:30 p.m. | 2:42 p.m. | 10:18 a.m. |
| Midnight | 3:12 p.m. | 10:48 a.m. |
Two things that table hides. 107 mg is a small coffee — the FDA puts a 12 fl oz brewed one at 113–247 mg, so a large sits nearer the right-hand column. And the pre-workout row lands before 11 a.m., which makes an afternoon gym session and an 11 p.m. bedtime arithmetically incompatible.
Half-life is the wrong number. Quarter-life is the right one.
Half-life is how long your body takes to clear half a dose. The American Academy of Sleep Medicine puts it at 3 to 5 hours; pharmacology reviews record 2.5–5 hours after a single measured dose. Almost every article stops there, and gets the timing wrong.
Bedtime is two half-lives out, not one. That’s the quarter-life — 6 to 10 hours, depending where you sit in the range — the point at which a quarter of the dose is still working.
| Hours since the cup | Half-lives elapsed | Still in you (5-h half-life) | From a 180 mg coffee |
|---|---|---|---|
| 5 | 1 — half-life | 50% | 90 mg |
| 10 | 2 — quarter-life | 25% | 45 mg |
| 15 | 3 | 12.5% | 22.5 mg |
| 20 | 4 | 6% | 11 mg |
A 3 p.m. coffee, starting at 180 mg, still has 45 mg circulating at 1 a.m. — roughly a green tea, arriving in the middle of the night.
Why your number isn’t anyone else’s
One liver enzyme, CYP1A2, accounts for more than 90% of caffeine metabolism — 15% of all the cytochrome P450 in your liver. How fast yours runs is the biggest reason two people react differently to the same cup. A second gene, ADORA2A, shapes the receptor side: its 1976T>C polymorphism modulates individual sensitivity to caffeine-induced insomnia.
Two things shift caffeine’s half-life more than genetics do, and neither is on the bag. Oral contraceptives nearly double it — 6.2 hours untreated against 10.7 treated. In the third trimester of pregnancy it stretches to 11.5–18 hours, back to normal within weeks of delivery. On the pill, an 11 a.m. cup is still nearly half live at an 11 p.m. bedtime.
It runs the other way too: smoking almost doubles caffeine metabolism, so quitting silently doubles your effective dose — and the sleep problem that follows gets blamed on withdrawal.
“But I fall asleep fine”
Twelve healthy normal sleepers took 400 mg of caffeine — two to three cups — at bedtime, three hours before, and six hours before. Even the six-hour dose cut objectively measured total sleep by more than an hour, and subjective ratings suggested the participants were unaware of it.
The meta-analysis shows where the damage sits. Falling asleep took only 9.1 minutes longer. But sleep efficiency fell 7.0% and time awake after falling asleep rose 11.8 minutes. Caffeine doesn’t mostly stop you getting to sleep — it thins the night after you’re in it.
If you sleep “enough” and still wake foggy, look here before adding hours or rearranging the bedroom.
The caffeine you forget about
FDA figures, per 12 fl oz:
- Brewed coffee: 113–247 mg. The familiar “95 mg a cup” assumes an 8-ounce cup nobody pours.
- Energy drink: 41–246 mg. Read the can, not the category.
- Black tea 71 mg, green tea 37 mg. Green tea is a third of a coffee, not zero.
- Soft drinks: 23–83 mg.
- Decaf: 2–15 mg per 8 fl oz. Small, real, additive across an evening.
Who should cut further
- Anyone on medication. The oral-contraceptive effect — 6.2 to 10.7 hours — shows how far one common prescription moves CYP1A2 clearance. Ask your provider, don’t guess.
- Pregnant or nursing. The FDA’s position is to ask your provider whether to limit caffeine. The third-trimester half-life makes an afternoon cup a different drink.
- Anyone prone to anxiety or reactive to small doses — you already know which side of ADORA2A you’re on.
- Anyone whose insomnia has lasted weeks. A provider conversation, not a timing problem — moving your coffee won’t reach a cause sitting somewhere else.
What to actually do
Do the free thing first: move the last dose, don’t cut it. The meta-analysis is a timing finding, not a volume one. Find your bedtime in the table, set a phone alarm, hold it two weeks.
If that’s not enough, shrink the dose before buying anything — blend regular with decaf, or drop a cup a day, since abrupt withdrawal brings a few rough days. Nothing on a supplement shelf beats an earlier cut-off. Then: sleep hygiene mistakes, better sleep naturally, how to fall asleep faster.
Questions people ask
I drink coffee at 6 p.m. and sleep fine. Doesn’t that settle it?
The twelve people in the Drake trial thought so too. Objectively they lost more than an hour of sleep from a dose six hours before bed, and their own ratings showed nothing. Falling asleep isn’t the measure that moved — efficiency and time awake were, and you don’t feel either.
Is decaf caffeine-free?
No. The FDA puts it at 2–15 mg per 8 fl oz. Two evening cups at the top of that range is half a green tea — not nothing, if your cut-off is tight.
I’m on the pill. How much earlier should I stop?
Half-life roughly doubles, 6.2 to 10.7 hours, so your quarter-life runs about 21 hours rather than 12. Make mid-morning your cut-off, not mid-afternoon, and judge it on two weeks.
How long before I know it worked?
Two weeks. Sleep varies enough night to night that two good ones prove nothing — and the effect you’re chasing, fewer minutes awake at 3 a.m., isn’t one you feel in a single night.
Sources
- The effect of caffeine on subsequent sleep: A systematic review and meta-analysis — Sleep Medicine Reviews, 2023;69:101764.
- Interindividual Differences in Caffeine Metabolism and Factors Driving Caffeine Consumption — Pharmacological Reviews, 2018;70(2):384–411.
- Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before Going to Bed — Journal of Clinical Sleep Medicine, 2013;9(11):1195–1200.
- Late afternoon and early evening caffeine can disrupt sleep at night — American Academy of Sleep Medicine, 2013.
- Sleep and Caffeine — Sleep Education, American Academy of Sleep Medicine.
- Spilling the Beans: How Much Caffeine is Too Much? — US Food and Drug Administration, 2024.
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