Caffeine Half-Life Calculator

This is exponential decay with a personal half-life. The gap between your cup and your pillow is measured in hours, wrapping past midnight where needed, and the dose is halved once for every half-life inside that gap. The metabolism selector supplies the half-life outright — seven hours if you describe yourself as sensitive, five for typical, three if caffeine barely touches you — and because it sits in the exponent rather than alongside it, that dropdown moves the bedtime figure further than anything else on the page. Two hundred milligrams taken at two in the afternoon before an eleven o'clock bedtime leaves about 57 mg on the normal setting, 82 mg on the sensitive one and only 25 mg on the fast one, the difference between a night flagged as disrupted and one that is not.

That flag is a single fixed cutoff of 40 mg; there is no gradual scale and no body-weight adjustment. The latest safe time works the same relationship backwards, solving for how many half-lives are needed to bring your dose down to that 40 mg line and stepping that many hours back from bedtime. One consequence catches people out: a dose already at or under 40 mg needs no decay time, so the hours-to-safe figure collapses to zero and the latest safe time is simply reported as your bedtime. Tolerance, pregnancy, medication that slows clearance, and the stimulating effect of caffeine taken hours earlier are all outside the model.