Caffeine Half-Life Calculator
See how much caffeine remains in your body hours after drinking it, and when it's safe to sleep without disruption.
What this calculates
Caffeine doesn't just vanish — it has a half-life of about 5 hours, meaning half is still in your system 5 hours after drinking it. A late-afternoon coffee can still impact sleep at midnight. This calculator shows the decay curve so you can time caffeine intake to avoid sleep disruption.
Formula & how it works
Remaining caffeine at time t = initial × 0.5^(t/half_life). Standard half-life is 5 hours but ranges 3–9 depending on genetics (CYP1A2 enzyme), pregnancy, smoking, and certain medications. For most people, caffeine taken 8 hours before bed still leaves ~20 % active at sleep onset.
Worked example
200 mg coffee at 2:00 PM. By 7:00 PM (5 hours): 100 mg remaining. By midnight (10 hours): 50 mg remaining. By 5:00 AM (15 hours): 25 mg — equivalent to a strong cup of green tea. Sleep impact is real even at 25 mg for sensitive sleepers.
Frequently asked questions
What's a 'safe' cutoff time?
Conventional advice: no caffeine after 2 PM if you sleep at 10–11 PM. Sensitive sleepers may need a noon cutoff. Heavy/regular coffee drinkers develop some tolerance to sleep effects but adenosine receptors still respond.
How much caffeine is in common drinks?
Espresso shot: 60–80 mg. Drip coffee 8 oz: 80–120 mg. Cold brew: 150–250 mg. Tea 8 oz: 30–50 mg. Coke: 30 mg. Energy drink: 80–300 mg. Pre-workout: 150–400 mg.
Does this account for genetics?
Use a longer half-life (7–9 hours) if you're a slow metabolizer — signs include feeling caffeine 'all day' from a single cup, jitters lasting hours, or a parent who's the same way.
Can I 'sleep off' caffeine?
Not really. The half-life isn't shortened by sleep — your liver works at the same rate. The body chemistry continues regardless of consciousness.