Caffeine Cutoff Calculator
Estimate a reasonable last-caffeine window from your bedtime, your intake, and your own sense of how strongly it affects you.
The method is published. The tool is not built yet.
A calculator that will not show its arithmetic is not evidence-first. The assumptions behind an estimate are the part you most need in order to judge whether the estimate is worth anything, so on Restadian they are published first and separately.
That is what this page is. It sets out what the tool will measure, what it will ask you for, what it will assume, and where the method is still being specified. When the tool ships it will do what is described here — and if the method changes while it is being built, this page changes with it.
There is nothing to run on this page. Nothing written below depends on the tool existing yet.
The method
What it will estimate
A window of time before bed within which further caffeine is more likely to still be affecting you, based on your reported intake and your reported sensitivity.
Note the shape of that claim carefully. It is a statement about likelihood across a wide range, not a statement about your bloodstream at a given hour.
What it will ask you for
- Your usual bedtime.
- Roughly what you drink, how much, and when.
- Your own read on how strongly caffeine affects you — whether an afternoon coffee is something you notice at night, or something you have never thought about.
That last input does real work. Self-reported sensitivity is imprecise, but it is considerably better information than the calculator would otherwise have.
What it will give back
A suggested window rather than a time, presented with its width visible rather than hidden. The explanation of why the range is as wide as it is will sit alongside the range itself, not in a footnote.
It will also show what changing your intake — rather than only its timing — would do to the estimate, so you can see whether the useful lever in your case is the amount, the hour, or neither.
The assumptions it will rest on
- Caffeine clearance follows a broadly predictable decay, with a highly variable rate. The shape of the curve is well described. The rate for any individual is not, and that variability is the dominant source of uncertainty in the output.
- Habitual intake shifts sensitivity. Regular consumption changes the picture, and the estimate will account for reported habit.
- A wide honest range beats a narrow false one. Where the method is uncertain, the range widens rather than the tool picking a midpoint and presenting it confidently.
Where the method is still being specified
The exact half-life range used, and how self-reported sensitivity is mapped onto it, are not finalised. Published values for caffeine half-life vary considerably, and selecting among them is a judgement we will document and source rather than bury in the code.
No numbers appear on this page yet for that reason.
What it will not do
- It will not guarantee, state, or imply that caffeine has cleared your system by any particular time.
- It will not produce a single “cutoff time” presented as a fact.
- It will not account for your medications, your health conditions, or pregnancy. Several medications meaningfully change caffeine metabolism, and a general calculator cannot model that.
- It will not advise you to increase your intake.
How to use it honestly
Take the suggested window, hold it for two weeks, and note what actually changes. If nothing does, caffeine timing was probably not your constraint — which is a genuinely useful result, because it means you can stop adjusting it and look somewhere else.
The calculator cannot distinguish between caffeine keeping you awake and something else keeping you awake while you happen to have had coffee. Only the experiment can do that.