How the Caffeine Clock calculator works

The model

Each drink is treated as a dose that is absorbed into the body at a first-order rate and eliminated at a first-order rate. For a drink of D milligrams taken at once, the amount in the body t hours later is:

C(t) = D × ka / (ka − ke) × (e^(−ke·t) − e^(−ka·t))

where ka is the absorption rate and ke the elimination rate. The amount rises while absorption outpaces elimination, peaks, and then falls away at the pace set by ke. Every drink’s curve is added up to give the total you see.

Absorption

The absorption rate comes from how long a drink takes to be 90 % absorbed (t90): ka = ln(10) / t90. Each item in the drink database carries its own t90: 175 of the 222 drinks use 45 minutes and most of the rest 35 or 40 minutes. Caffeine usually peaks in the blood 30 to 120 minutes after you drink it.

Elimination and half-life

The elimination rate follows from your caffeine half-life: ke = ln(2) / t½. The default is 5 hours, inside the 3 to 6 hours reported for most adults. Half-life differs between people: smoking shortens it and oral contraceptives lengthen it, which is why you can set your own.

Sipping versus gulping

A drink you finish in one go is modelled as a single dose. A drink you sip is modelled as a steady intake spread over the time you took to finish it (the app’s default is 10 minutes), which gives a lower, later peak than drinking the same amount at once.

Sleep threshold and bedtime

The sleep threshold (100 mg by default) is the level you want to be under at bedtime. The European Food Safety Authority notes that single doses of 100 mg may affect sleep duration and patterns in some adults, particularly close to bedtime. The calculator checks the amount at your next bedtime against this threshold and finds the moment you drop below it.

Daily limits for context

For most healthy adults the FDA cites 400 mg a day as an amount not generally associated with negative effects, and EFSA reaches the same figure, with single doses up to 200 mg raising no safety concerns. For pregnant and breastfeeding women EFSA’s figure is up to 200 mg a day.

Limits of the model

The model describes a typical adult. How quickly caffeine is cleared differs from person to person (smoking and oral contraceptives are two documented reasons), and the caffeine content of the same drink varies between recipes and batches. Caffeine Clock’s numbers are estimates to help you plan, not a medical measurement or medical advice.

Sources

1. U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine Is Too Much? https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much
2. European Food Safety Authority. Caffeine (topic page summarising the 2015 Scientific Opinion on the safety of caffeine). https://www.efsa.europa.eu/en/topics/topic/caffeine
3. 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. 2013;9(11):1195-1200. https://doi.org/10.5664/jcsm.3170
4. Mort JR, Kruse HR. Timing of blood pressure measurement related to caffeine consumption. Annals of Pharmacotherapy. 2008;42(1):105-110. https://doi.org/10.1345/aph.1k337
5. Grzegorzewski J, Bartsch F, Köller A, König M. Pharmacokinetics of caffeine: a systematic analysis of reported data for application in metabolic phenotyping and liver function testing. Frontiers in Pharmacology. 2021;12:752826. https://doi.org/10.3389/fphar.2021.752826