About

Caffeine Curfew is built and written by Toni Ristola. It began as a question with an annoying answer: every guide says to stop drinking coffee six hours before bed, none of them asks what you drank or when, and the arithmetic to find out is short enough that nobody should have to do it by hand.

Six hours is not wrong so much as it is a single answer to a question with three variables. It is the right number for one dose, one metabolism and one bedtime, and it goes on being quoted at everyone else. The whole site is an attempt to replace that one number with your three.

What this is

Arithmetic over published averages. Caffeine decays exponentially, halving roughly every five hours in a typical adult; sleep runs in cycles of roughly ninety minutes. The calculator applies both to the numbers you give it and shows the working, including the parts that are uncertain. Every caffeine figure in the drink table names the record it came from, and the four that are estimates say so rather than borrowing the authority of the twelve that are not.

Everything runs in your browser. There is no account, no server that sees your bedtime, and no model of you being built in the background — the calculation is short enough to do on the page, so it is done on the page. That is also why it still works with the network off.

Where the numbers come from

Three sources carry most of the weight, and each page lists the ones it actually leans on rather than a shared bibliography padded to look thorough.

• The claim that a late dose costs measurable sleep without being noticed comes from Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine. — a controlled trial in which 400 mg taken six hours before bed cost about an hour of sleep, and participants did not reliably report feeling it.
• The general position on afternoon and evening caffeine is the American Academy of Sleep Medicine's.
• The three-to-seven-hour half-life range, and the fact that it is a range rather than a figure, follows the Institute for Scientific Information on Coffee summary of caffeine metabolism.

The caffeine content of individual drinks is the part most likely to be wrong, and the site says so where it matters: a 240 ml filter coffee runs anywhere from about 70 to 140 mg depending on bean, roast, grind and contact time, and the 95 mg the calculator defaults to is a central value from that range rather than a measurement of the cup in front of you. Every preset in the drink table names its record; the estimates are labelled as estimates.

What this is not

It is not a measurement of you, and it is not medical advice. No clinician has reviewed it. Toni Ristola is not a doctor, a sleep scientist or a pharmacologist — the credential behind this site is that the model is stated openly enough for anyone who is one to check it. Half-life varies from roughly three to seven hours between healthy adults, and medication, pregnancy and health conditions move it further. If caffeine or sleep is causing you real trouble, that is a conversation for someone qualified to have it.

It also says nothing about safety. It estimates one thing — how much caffeine is likely to still be in you at bedtime — and nothing it produces bears on driving, operating machinery, or a cardiac question.

How to check it, and how to correct it

The model is deliberately small enough to audit. A dose decays as dose × 0.5^(t ÷ half-life), the total is that sum across everything you have drunk, and the cutoff is the point where one more drink would decay to exactly the headroom you have left. Every default the calculator uses — the five-hour half-life, the 50 mg threshold, the 95 mg cup — is adjustable and visible, so a disagreement with the result can always be traced to a disagreement about an input.

When something here is wrong, the fix is a correction, not a quiet edit. The last-updated date on every page is derived from the site's own version history rather than typed by hand, so it cannot flatter the content by claiming a freshness that did not happen. There is no review date and no reviewer badge, because there has been no review. If you have a better source for any figure on the site, write in — that is the single most useful thing anyone sends.

Why 9 languages

The advice this site argues with is quoted in every language, and the arithmetic that replaces it does not care which one you think in. The pages are written for each language rather than run through a translator: the questions answered on the German pages are the ones German readers actually ask, which are not the same questions, and the drinks in the table differ by market. It is the same model everywhere and, deliberately, not the same page.

The site itself

It stores nothing to work out your answer — the whole calculation happens in your browser and your settings travel in the page address, which is why share links work before you have made any choice about cookies. The privacy and cookies pages set out exactly what does and does not touch your device, including the one thing that does if you ask for it.

It is paid for by advertising, and by affiliate links where a page recommends something you can buy. Those links are marked, and the affiliate disclosure says which ones they are and what the site earns. Nothing on this site is recommended because it pays, and no advertiser has any say in what the pages say — but you should not have to take that on faith, which is why the disclosure exists.

The calculator is also free to embed. It goes into a page as one iframe, carries no ads, no analytics and no cookies inside the frame, and asks only for a credit line underneath.

Contact

Corrections are welcome, and a wrong number is worth reporting. Email hello@caffeinecurfew.com, or see the contact page for what is and is not worth writing about.