Tool · Sleep & Recovery

Caffeine Half-Life Plotter

What "one last coffee at 4 pm" actually does to your blood at 11 pm. The curve, the bedtime impact, and a realistic last-call time.

Caffeine (mg) Time you had it

Your metabolism Planned bedtime

Remembers last drink on this device.

At bedtime

— mg

Sleep threshold

50 mg

Roughly the level where most adults stop feeling stimulated. Not a guarantee.

Last call time

—

To land below 50 mg by bedtime.

How to use it

Pick a preset or enter the milligrams and the time you had it.
Choose your metabolism. Default 5 h is the population average — slow and fast are real and matter more than people think.
Tell it your bedtime. The plot goes 24 hours from intake, with bedtime marked.
Read the "At bedtime" number — that's what's still in your blood when you try to sleep.

When to reach for it

• You keep "cutting off at 2 pm" but still can't sleep — see why.
• You're trying to fix sleep and your afternoon habit is the suspect.
• You're switching from drip to cold brew and want to know it's not a free upgrade.
• You're pregnant, on certain meds, or just slower at processing it.

Reading your results

Under 50 mg at bedtime is the working target — that's where most healthy sleepers stop feeling the stimulant effect. Above 100 mg at bedtime reliably delays sleep onset in adults who are sensitive. The plot also shows when you'll cross back under 50 mg so you can plan your last cup.

What this actually does

Caffeine pharmacokinetics are well-mapped: after ingestion, plasma concentration rises within 30–60 minutes and then declines exponentially, with a half-life that varies by person. The "average adult" half-life is around 5 hours, but published studies (e.g. Fredholm et al., 1999) put it anywhere from 3 to 9 hours depending on genetics, pregnancy, smoking, liver function, and a few common medications (the oral contraceptive pill roughly doubles it; some SSRIs slow it).

The plot is a first-order exponential decay: mg(t) = mg₀ × 0.5^(t / half-life). It's a model, not a measurement. Your real curve will differ by 20–30%, especially in the first hour while absorption is still happening.

Who it's for

Anyone whose sleep is "fine, except for the falling-asleep part". People who feel they've already tried cutting off caffeine and it didn't help — usually they cut to a different number, not actually under 50 mg. People who want a concrete reason to switch from a 3 pm drip to a 10 am drip.

If you're using caffeine to manage a medication side-effect, an ADHD diagnosis, or a chronic condition, this calculator is not a substitute for the conversation with your prescriber.

Limitations

The milligrams in a "cup" vary wildly. A "drip coffee" at one cafe is 95 mg; at another it's 220. We use the USDA reference values for the presets — they're a reasonable midpoint, but real drinks wander ±30%. If you want precision, weigh your beans and dose in grams.

Caffeine also has a small wake-promoting effect via adenosine receptors that lingers past the blood level, so even when plasma caffeine drops below 50 mg, you may still feel slightly alert. Most adults don't notice it; some do.

How to read the chart

The curve falls by half every "half-life" along the x-axis. The dashed line is 50 mg — the working sleep threshold. The orange band is your bedtime. If the curve crosses bedtime below the dashed line, you're probably in the clear for sleep; above, plan to bring intake earlier or reduce the dose.

FAQ

Is decaf actually caffeine-free?

No. A typical decaf has 2–15 mg per cup. Fine for most people, not zero. If you're extremely sensitive or pregnant, even decaf adds up across a day of switching.

What about pre-workout or caffeine pills?

Same model, just type the milligrams directly. Most pre-workouts contain 150–300 mg per scoop. Caffeine pills are usually 100 or 200 mg each.

I'm on the pill. What half-life do I pick?

Combined oral contraceptives roughly double the half-life in some users. If your "before the pill" half-life was 5 h, expect closer to 7–8 h on it. Use "slow metaboliser" as a starting point.

What if I have caffeine and then nap?

A 20-minute "coffee nap" works because caffeine takes about 20 minutes to peak in the blood. If your nap is longer than that, you wake up mid-ride and feel worse. The plot assumes you stayed awake; if you dozed, your body absorbed less.

What are the common mistakes when using Caffeine Half-Life Plotter?

Most common mistakes come down to four habits: entering 20 when the field expects 0.2 because the field is a percentage; leaving a field blank and accepting the default without checking what it stands for; changing one input and reading a result that was calculated before the change; and quoting the output as an exact figure when the page describes it as a reference estimate. Press calculate again after any edit, and treat the number as a reference point rather than a final answer.

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