Caffeine Half-Life Calculator

Why caffeine half-life matters for sleep

Caffeine works by blocking adenosine receptors. Adenosine is a brain chemical that builds up while you're awake and creates the pressure to sleep. Blocking it doesn't remove tiredness — it masks it. When the caffeine clears, the backlog of adenosine floods the receptors at once.

The textbook half-life for caffeine in healthy adults is about 5 hours, meaning every 5 hours half of what remains is metabolized. Even at "negligible" 12–25 mg residual levels, polysomnography shows reduced deep sleep and more nighttime wake-ups for sensitive people.

Individual variation: CYP1A2

Caffeine is metabolized almost entirely by the CYP1A2 liver enzyme. Genetic variants split adults into roughly two populations:

• Fast metabolizers — half-life around 3–4 hours. Often tolerate evening coffee without sleep disruption.
• Slow metabolizers — half-life 6–8+ hours. A noon coffee can still affect sleep onset 12 hours later.

Other factors that lengthen half-life: oral contraceptives (nearly double it), pregnancy (up to 15 hours in third trimester), liver disease, some SSRIs. Smoking shortens half-life — quitters often need to cut their coffee intake to avoid jitters.

Recommended caffeine cutoff

A 2013 study in the Journal of Clinical Sleep Medicine tested 400 mg caffeine doses at 0, 3, and 6 hours before bed. All three doses caused measurable sleep disruption — even the 6-hour-before dose reduced total sleep by over an hour. Conservative guidance: no caffeine within 8–10 hours of bedtime, especially for slow metabolizers and people with sleep complaints.

How the formula works

Standard first-order decay: remaining = dose × 0.5^(hours / half-life). So 200 mg with a 5-hour half-life leaves 100 mg after 5 h, 50 mg after 10 h, 25 mg after 15 h. The calculator above runs this hour by hour and flags how much is still active at your chosen bedtime.

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