Caffeine Half-Life & Cutoff Calculator

Caffeine's half-life ranges from about 4 to 7 hours depending on how fast you metabolise it. Add what you've already drunk today to find the last time your next drink still leaves under 50 mg in your system at bedtime.

Caffeine Sensitivity

Not sure? If an afternoon coffee keeps you up at night, pick Slow.

Drinks so far today

Earlier caffeine is still in your system and pulls your cutoff earlier.

Pick your next drink

Last safe 100 mg drink

2:49 PM

Calculated so that under 50 mg of caffeine is still circulating at bedtime, counting today's drinks.

Today's drinks alone leave about 14 mg at bedtime.

50 mg
7:30 AMBed · 11:00 PM

Solid: caffeine from today's drinks. Dashed: with one more drink at your cutoff, landing on the 50 mg line at bedtime.

Stop guessing your cutoff.

Here you type your drinks in. ARC does the same math from every drink you log in one tap, moves your last safe coffee the moment you log a cup, shows it as a live countdown, and tells you plainly when no cup fits any more.

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<iframe src="https://arcapp.sbs/tools/caffeine-calculator" width="100%" height="600px" frameborder="0"></iframe>
<p>Caffeine calculator by <a href="https://arcapp.sbs/tools/caffeine-calculator">ARC</a></p>

The Science Behind the Calculator

Caffeine has a half-life of about 4 to 7 hours depending on how fast you metabolise it (around 5.5 on average). For an average metaboliser, about half of a coffee is still circulating 5 to 6 hours after you finish it, blocking adenosine receptors and holding off the sleep pressure that should be building.

Why is my cutoff time so early?

The calculator aims for under 50 mg of caffeine still circulating at bedtime, the same line the ARC app uses. If you have a 200 mg energy drink at 3:00 PM, you'll still have roughly 50 to 90 mg circulating at 11 PM (about 73 mg for an average metaboliser), above the 50 mg line. The calculator does the exponential decay math for you, so you're under 50 mg by the time you go to bed.

How does this affect my circadian rhythm?

Caffeine doesn't remove sleep pressure; it masks it by blocking adenosine receptors. When it wears off, the adenosine that built up is still there, which is the crash. Caffeine late in the day can also make it harder to fall asleep and lighten your sleep, even on nights you drift off fine.

Frequently asked questions

What is the half-life of caffeine?

Roughly 4 to 7 hours depending on the person, around 5 on average. Genetics, pregnancy, smoking and some medications all change it. The calculator lets you pick fast (4 h), average (5.5 h) or slow (7 h), the same three options the ARC app asks about.

When should I stop drinking coffee before bed?

There's no universal cutoff. For one average cup (about 100 mg) and an average metaboliser, caffeine takes roughly five and a half hours to fall below 50 mg. A bigger cup, a slower metabolism, or an earlier coffee still in your system pushes that earlier.

How much caffeine is still in my system at bedtime?

Take a 200 mg energy drink at 3 PM and a bedtime of 11 PM. Eight hours later you'd still have about 50 mg circulating if you clear caffeine fast, about 73 mg if you're average and about 91 mg if you're slow.

Does my morning coffee affect my cutoff?

Yes. Caffeine from earlier drinks is still in your system, so it uses up part of the room you have before bedtime. Two coffees at 8:20 and 10:45 can pull the last safe time for a third cup from about 3 PM to about 11 AM. Add every drink you've had today to see your real cutoff.

Why does the calculator aim for under 50 mg at bedtime?

It's the line the ARC app uses: a practical threshold rather than a hard biological cliff. If even a small afternoon coffee keeps you awake, choose the Slow setting to build in more margin.

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