The short answer
Caffeine has a half-life of about 5 hours, so half of it is still in your system 5 hours after your cup — and about a quarter is still active at 10 hours. You feel it within 15 minutes, it peaks 30–60 minutes after drinking, and it tapers off slowly from there rather than switching off.
That slow tail is why an afternoon coffee can still affect your sleep at midnight, even if you no longer feel wired.
What "half-life" actually means
Half-life is the time your body takes to clear half of a dose. With a ~5-hour half-life, a 200 mg coffee (about two cups) breaks down roughly like this:
• At 5 hours: ~100 mg left.
• At 10 hours: ~50 mg left.
• At 15 hours: ~25 mg left.
So caffeine doesn't "wear off" at a fixed time — it halves, then halves again. A big or late dose leaves a meaningful amount active well into the night. You can map your own dose over time with the caffeine half-life calculator.
What changes how long it lasts
The 5-hour figure is an average; your personal half-life can range from about 1.5 to 9+ hours depending on:
• Genetics. Variations in the CYP1A2 liver enzyme make some people "fast" and others "slow" caffeine metabolizers.
• Pregnancy. Caffeine clears much more slowly — its half-life can more than double, especially in the third trimester.
• Smoking. Speeds caffeine clearance (roughly halves the half-life); quitting slows it back down.
• Medications & liver health. Some drugs (certain antibiotics, oral contraceptives) slow caffeine breakdown, extending its effects.
Why it matters for sleep
Because caffeine lingers, timing matters more than most people think. Even if you feel fine, residual caffeine can reduce deep sleep. If coffee ever keeps you up, see when to stop drinking coffee before bed to find your personal cutoff, and check your daily total against safe limits in how much caffeine is too much.