Halfwake · free tool
Caffeine half-life calculator
Estimates blood caffeine concentration from a dose, your body weight, and your metabolic profile — and reports when it falls back below the 1.5 mg/L sleep threshold.
Clears the 1.5 mg/L sleep threshold in —.
How it works
Caffeine follows first-order elimination kinetics. After a dose of
D mg, blood concentration C(t) in mg/L is approximated by
C(t) = (D / (Vd · weight)) · 2−t/t½,
where Vd is the volume of distribution (≈ 0.5 L/kg
for caffeine) and t½ is the half-life in hours. Half-life
varies from roughly 3 hours in fast metabolisers to 9+ hours in slow
metabolisers, pregnancy, or with certain medications.
What the sleep threshold means
Blood caffeine above roughly 1.5 mg/L is enough to measurably delay sleep onset and reduce slow-wave sleep in healthy adults. It is a useful target, not a clinical cutoff.
Want it tracked daily?
Halfwake logs every drink, fits the same model to your real intake through the day, and shows the curve in context.
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