Sources and calculation method
The science behind the caffeine estimate
SlumberBrew is designed to show a transparent estimate rather than pretend it can measure caffeine in your body. Here is what the model does, where its numbers come from, and where it stops.
What the model estimates
When you log a drink, SlumberBrew estimates how much caffeine may be in your system now, how much may remain at bedtime, and when the level may fall back under the bedtime target you chose. It combines the drinks you log instead of looking only at your last cup.
How half-life works
Caffeine is modelled with a half-life: the time it takes for roughly half of the amount in your system to clear. If the half-life is five hours, 200mg becomes about 100mg after five hours and about 50mg after ten. The app also models a short absorption period, because a drink does not appear in the body as a finished number the instant you swallow it.
Why the answer is not one exact number
The caffeine in a named drink varies with the beans, serving size, recipe, and brewing method. That is why the drink catalogue uses typical values and the brew calculator gives a range based on the coffee weight or water-to-coffee ratio you enter. A range is more honest than a precise-looking number that the cup did not justify.
Why the estimate can differ between people
People clear caffeine at different rates. Age, pregnancy, medication, smoking, genetics, and other factors can change the half-life. SlumberBrew lets you adjust that setting so the model does not quietly assume that one average describes everybody.
What it cannot tell you
It cannot measure your blood caffeine level, diagnose a sleep problem, or prove that caffeine caused a particular night. The browser tracker compares your log with the bedtime target you set. It does not read your sleep data. The iPhone app can optionally read sleep recorded in Apple Health so you can inspect the pattern yourself, but that still does not turn the estimate into a medical assessment.
SlumberBrew is not a medical device and none of its output is medical advice. If you have a health condition, take medication, are pregnant, or are worried about caffeine or your sleep, talk to a doctor.
Published sources
- Blanchard & Sawers (1983) for half-life range, time to peak, and bioavailability.
- Nehlig (2018) for why caffeine metabolism varies between people.
- Drake and colleagues (2013) for the relationship between evening caffeine and sleep.
- EFSA (2015) for general caffeine intake guidance.
About SlumberBrew
SlumberBrew is made by Cael Jones. The aim is to make caffeine timing easier to see without requiring an account or sending your drink log to a server. The calculator is a practical guide built from published averages, not a promise of what your body must do.
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