📉 What is “active caffeine”?
La Active caffeine (or residual caffeine) is the amount of caffeine, in milligrams, still in your body right now. It isn't the total you've drunk since this morning: it's what remains after your body has started clearing it.
When you drink a coffee, that amount rises quickly, reaches a peak, then falls away gradually. PauseCafé plots this curve for each of your drinks and adds them together, which gives the curve you see on the home screen, on your watch and in the widget.
What you drink
Each drink adds its dose of caffeine to the total in your body.
The rise
Caffeine is absorbed quickly: the peak is reached in under an hour.
The fall
After that, your body clears it gradually, hour by hour.
⏳ How it's calculated
PauseCafé builds on the pharmacokinetics of caffeine : how the body absorbs and then clears it. Two key figures, taken from the scientific literature:
- Absorption. Caffeine enters the bloodstream quickly. The amount in your body reaches its maximum (the “peak”) around 45 minutes after drinking — the reported range is 30 minutes to 2 hours. Not to be confused with the “15 to 30 minutes” often quoted: that describes the onset of perceived effects, not the peak amount.
- Clearance. After that, the amount falls according to a half-life of about 5 hours in a healthy adult (the average value used by EFSA). In other words, every 5 hours, roughly half is left.
- Adding it up. PauseCafé applies this calculation to each of your drinks and sums the results. What comes out is the active caffeine curve: the total estimated amount in your body, minute by minute.
Under the bonnet, this is a standard one-compartment pharmacokinetic model (a rise phase, a fall phase). It's deliberately simple: the aim isn't to simulate your exact physiology, but to give you a useful, readable benchmark — “how much is left, and where will I be tonight”.
🔬 The model in detail — for the curious
PauseCafé uses a one-compartment model with first-order absorption — the Bateman function. For a drink of dose D taken at time 0, the amount still in the body at time t (in hours) is:
The curve you see is the sum of this contribution for each of your drinks. You can try the calculation in your browser without installing the app. The constants:
- ke (elimination) = ln(2) / 5 h ≈ 0.139 h−1 — that is a 5 h half-life (healthy adult average, EFSA).
- ka (absorption) ≈ 4.9 h−1 — calibrated for a peak (Tmax) ≈ 45 min after drinking (range 30 min – 2 h).
- Bioavailability ≈ 100% — oral caffeine is almost entirely absorbed.
This is an estimated amount in mg in the body, based on a population average — not a blood concentration (mg/L), nor an individual measurement. Indicative, wellness, not medical.
🌙 The sleep threshold
Below a certain level, caffeine no longer really gets in the way of falling asleep. PauseCafé calls this benchmark the sleep threshold.
🌙 How it's set
By default the threshold is 100 mg — the benchmark highlighted by EFSA for caffeine's impact on sleep. If you enter your weight in the app, the threshold is refined (around 1.4 mg per kilo). Looking at your curve in the evening, you can estimate when you drop back below that threshold, and aim for a calmer bedtime.
This threshold is shown as a green zone under the curve, with a vertical marker at your bedtime — on iPhone, the widget and Apple Watch: at a glance you see how much caffeine will be left when you sleep.
🔤 The sleep impact grade (A → G)
On the detail screen, a grade in the style of an energy label (A to G, green to red) sums up the estimated impact of your caffeine on your sleep. It compares how much caffeine will be left at your bedtime against your sleep threshold :
- A — Sleep protected : under a quarter of the threshold.
- B — Very favourable : under half the threshold.
- C — Favourable : under three quarters of the threshold.
- D — Borderline : right at the threshold.
- E — Worth watching : up to 1.5× the threshold.
- F — Unfavourable : up to 2.25× the threshold.
- G — Very unfavourable : beyond that.
The closer the letter is to G, the more caffeine is left at bedtime. It's a visual benchmark, indicative and not medical, based on the same estimate as the curve.
🧭 The practical markers
Beyond the curve, the detail screen shows markers you can act on:
- Last recommended coffee — the time before which to have your last coffee if you want to be below your threshold at bedtime, or a “Limit reached” message if one more would push you over.
- Back below the threshold — the estimated time at which your active caffeine drops back below your sleep threshold.
- Today's mg/kg ratio — your total caffeine today relative to your weight, compared with the EFSA benchmark of 5.7 mg/kg/day (your reference daily dose given your weight, that is ≈ 400 mg for 70 kg). Your weight stays on your device, never sent anywhere.
📲 Where to find it
On iPhone
A card on the home screen shows your active caffeine and its curve.
On Apple Watch
Shown in the Watch app, and as complications on your watch face: today's caffeine and active caffeine. The rectangular one plots the curve up to your bedtime.
In the widget
Your active caffeine, visible straight from the home screen.
🧮 An estimate, not a measurement
This is the most important point. The figures shown are an estimate based on a population average, not a measurement of your blood. How fast you actually clear caffeine depends on factors the app cannot measure :
la genetics (the CYP1A2 enzyme, “fast” or “slow” metabolisers), smoking (which speeds clearance up), oral contraceptives and pregnancy (which slow it down), age, or certain medications. That's why PauseCafé does not personalise clearance speed by body weight: it would be false precision.
In the same way, “active caffeine” describes an estimated amount present in the body — not an intensity of effect, nor a blood concentration in mg/L. Two people with the same amount won't necessarily feel the same thing.
⚠️ Wellness information
PauseCafé is a wellness app. The information provided is for guidance and has no medical value: it is neither a medical device nor medical advice. For any question about your sleep or your health, consult a healthcare professional.
Sources: EFSA, Scientific Opinion on the safety of caffeine, 2015 (EFSA Journal 2015;13(5):4102) — average half-life ≈ 5 h in a healthy adult, sleep impact threshold ≈ 100 mg, daily dose of no concern ≈ 400 mg (that is ≈ 5.7 mg/kg/day) and ≈ 200 mg per single dose. Absorption pharmacokinetics (peak ≈ 30 min – 2 h) from the pharmacological literature. EFSA opinion →
See your caffeine, in real time.
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