Caffeine Half-Life Calculator.
Plot your bloodstream caffeine decay curve across the day and find the exact time you need to stop drinking coffee to protect your sleep.
Medical Disclaimer
This calculator uses population-average pharmacokinetic constants. Individual clearance varies with liver function, medication, pregnancy stage, and oral contraceptive use. For clinical guidance, consult a physician.
How the Math Works
Caffeine clearance follows first-order elimination — the body removes a fixed fraction of whatever is in the bloodstream each unit of time. Half the dose disappears every half-life.
- 01Decay constant. k = ln(2) / half-life. For an average metabolizer (t½ = 5.5h), k ≈ 0.126 per hour.
- 02Single dose decay. C(t) = D × exp(-k × Δt). A 100 mg dose 11 hours later is 100 × exp(-0.126 × 11) ≈ 25 mg.
- 03Superposition. For multiple doses, total caffeine is the sum of each individual decay curve. The calculator adds them at every quarter-hour from 0 to 36 hours.
- 04Bedtime threshold. The 50 mg horizontal line marks the published level above which slow-wave sleep is measurably suppressed.
- 05Cutoff time. Solving C(t) = 50 backward from bedtime gives the latest time you can have caffeine without exceeding the threshold.
Frequently Asked Questions
How long does caffeine stay in your body?+
Caffeine has an average half-life of about 5.5 hours in healthy adults, meaning half the dose is cleared every 5.5 hours. A standard 200 mg dose at noon drops to ~100 mg by 5:30 PM, ~50 mg by 11:00 PM, and ~25 mg by 4:30 AM.
Why does the calculator support five metabolizer types?+
The CYP1A2 enzyme drives ~95% of caffeine breakdown, and genetic variants change its activity. Pregnancy roughly doubles the half-life (ACOG guidance), while smoking induces CYP1A2 and shortens the half-life to ~2.5 hours.
What is the 50 mg bedtime threshold based on?+
Sleep research shows that doses as low as 20–50 mg in the bloodstream near bedtime can suppress slow-wave (deep) sleep and increase sleep onset latency. The 50 mg line is a conservative cut-off; sensitive sleepers may need an even lower target.
What math does the calculator use?+
It applies first-order elimination kinetics: C(t) = D × exp(-k × Δt), where k = ln(2) / half-life. Multiple doses are summed via the superposition principle — each dose decays on its own curve, and total bloodstream caffeine is the sum at any given time.
How accurate is the prediction?+
The model is a population-level estimate. Real-world clearance varies with liver function, oral contraceptives, certain medications, and food. Treat the curve as a planning guide, not a precise blood measurement.
How can I avoid caffeine ruining sleep?+
Work backwards from bedtime: if your half-life is 5.5 hours, cutting off at 8–9 hours before bed keeps bedtime levels below 50 mg for most moderate-dose coffee drinkers. The calculator finds your personal cutoff.