How the caffeine cutoff calculator works
The half-life, and why afternoon coffee is not free
Caffeine clears with a half-life around 5 hours (3.5 in fast metabolizers, 7+ in slow ones and with some medications). A 3 pm coffee leaves a 200 mg drinker with ~100 mg at 8 pm and ~50 mg at 1 am — enough to lengthen sleep latency and shave deep sleep even in people who report falling asleep normally. The subjective "caffeine does not affect me" is usually a statement about falling asleep, not about sleep quality.
Sensitivity is real and partly genetic
CYP1A2 variants make some people near-fast metabolizers (cutoff barely matters) and others slow (even noon doses linger). Anxiety, jitters, and afternoon crashes are the subjective signatures. The sensitivity input adjusts the half-life — honest about being a self-assessment, useful enough to beat ignoring the variable.
Practical cutoffs that work
The research floor: ~6 hours before bed preserves most subjective sleep; the cautious standard: 8–10 hours. Decaf (2–5 mg), herbal tea, and the ritual-without-the-molecule options carry the afternoon. Total daily dose still matters — up to ~400 mg/day is the safety line for most healthy adults.