Caffeine Sleep Impact Calculator
Estimate how much caffeine remains active in your body at bedtime based on when you last consumed it and your personal sensitivity. Use it to decide your caffeine cutoff time so it doesn't wreck your sleep.
About this calculator
Caffeine follows first-order exponential decay in the bloodstream. Its half-life — the time for concentration to halve — ranges from 3 to 7 hours depending on genetics, liver enzyme activity, and medications. The formula used is: Remaining Caffeine = caffeineAmount × 0.5^((bedtime − consumptionTime) / sensitivity), where sensitivity represents your personal half-life in hours. For example, 200 mg consumed 5 hours before bed with a 5-hour half-life leaves 100 mg active at bedtime — enough to delay sleep onset and suppress slow-wave sleep. Even 25–50 mg of residual caffeine has measurable effects on sleep architecture. This is why a 3 PM coffee can still disrupt sleep at 11 PM for people with a longer half-life.
How to use
You drink 200 mg of caffeine at 2:00 PM and plan to sleep at 11:00 PM — that's 9 hours later. Your caffeine sensitivity (half-life) is 5 hours. Step 1: exponent = 9 ÷ 5 = 1.8. Step 2: 0.5^1.8 ≈ 0.287. Step 3: 200 × 0.287 ≈ 57 mg remaining at bedtime. Since 50+ mg is enough to measurably impair sleep, you may want to move your last coffee earlier or switch to a smaller dose.
Frequently asked questions
What is the half-life of caffeine and why does it vary between people?
The average caffeine half-life is about 5 hours, but it ranges from 1.5 hours in heavy smokers to over 9 hours in pregnant women or those taking certain medications like oral contraceptives. The primary enzyme responsible, CYP1A2, is genetically variable — roughly half the population are 'slow metabolizers' who clear caffeine much more slowly. This explains why one person can drink espresso after dinner and sleep soundly, while another is wide awake past midnight after an afternoon latte.
How much caffeine at bedtime actually disrupts sleep?
Research published in the Journal of Clinical Sleep Medicine found that 400 mg of caffeine consumed 6 hours before bed reduced total sleep time by more than 1 hour. Even 100 mg taken 3 hours before bed measurably disrupted slow-wave sleep. The threshold for noticeable sleep disruption varies, but most sleep scientists recommend keeping residual caffeine below 25–50 mg at bedtime. Slow-wave sleep is particularly vulnerable, which impairs next-day physical and cognitive recovery even if total sleep duration seems adequate.
When should you stop drinking coffee to avoid affecting your sleep?
A commonly cited guideline is to stop caffeine intake at least 6 hours before bed, but this is conservative for fast metabolizers and insufficient for slow ones. Using your personal half-life with this calculator gives a more precise cutoff time. For a typical 200 mg cup with a 5-hour half-life, you'd need to stop 8–9 hours before bed to drop below 50 mg residual. If you have trouble falling asleep, pushing your caffeine cutoff to noon is a practical starting point regardless of half-life.