< JMS / TOOLS

Half-life / balance / cutoff

Caffeine Planner

Say what level you want to hold and when you need to sleep. It solves backwards from there — the times, the doses, and the point in the afternoon where one more cup costs you the night. Every dose stays editable, so you can plan the day or just log the one you're already having and see where it lands.

mg in you

Wake up
In bed by
First dose
hours

Five hours is the usual figure for a healthy adult, but it's the number that changes everything downstream — smoking roughly halves it, hormonal birth control and pregnancy can double or triple it, and some people are just slow metabolizers. If caffeine at 2 PM keeps you up, your half-life is longer than you think.

Fine print & limits
Daily ceiling
Bedtime limit
Round doses to
Smallest worth taking
Minimum gap
Absorption

400 mg a day is the FDA's figure for healthy adults; 200 mg is the usual ceiling in pregnancy. The realistic setting models caffeine actually being absorbed from your gut, so a dose ramps up over about 45 minutes and never quite delivers its full number into your bloodstream at once — that's why the planned doses run slightly larger than a flat half-life sheet would suggest.

Your settings and doses are saved on this device (needs cookies / local storage).

// Note: This is a pharmacokinetic model, not medical advice. It uses first-order elimination — the same exponential decay your liver actually runs on — but the half-life it uses is the one you set, and the real one varies enormously between people and even day to day. Caffeine interacts with a long list of medications, and if you're pregnant, have a heart rhythm problem, an anxiety disorder, or reflux, the right number for you is one a doctor gives you, not one a web page does.

CSV gives you the half-hour balance table, formulas intact, if you'd rather keep working in a spreadsheet. "Add to calendar" downloads a standard .ics file — a week of daily reminders, one per dose, with the amount in the title.

jeremymsparks.com