Caffeine Half-Life Calculator

How caffeine metabolism works

Caffeine is metabolized primarily by the CYP1A2 enzyme in the liver. The average half-life of caffeine is about 5 hours, meaning that 5 hours after consuming 200mg, about 100mg remains in your system. After 10 hours, 50mg remains. After 15 hours, 25mg remains. This is why a 2pm coffee (100mg) still has ~25mg in your system at midnight, enough to disrupt sleep in sensitive individuals. But the 5-hour average masks enormous individual variation driven by genetics, medications, and lifestyle factors.

CYP1A2 genetics, fast vs. slow metabolizers

The CYP1A2 gene has a common variant (rs762551) that determines whether you are a "fast" or "slow" caffeine metabolizer. About 50% of the population are slow metabolizers (A/C or C/C genotype) with a half-life of 7-8 hours. The other 50% are fast metabolizers (A/A genotype) with a half-life of 2-3 hours. Slow metabolizers are much more likely to experience caffeine-related sleep disruption, anxiety, and palpitations. They also have higher risk of caffeine-related heart attacks (per a 2006 study). If you find that a 2pm coffee disrupts your sleep, you are likely a slow metabolizer.

Factors that change caffeine half-life

Oral contraceptives: Estrogen inhibits CYP1A2, doubling caffeine half-life from ~5 hours to ~10 hours. Women on birth control pills are effectively all slow metabolizers.

Pregnancy: Caffeine half-life increases progressively through pregnancy, reaching 11-15 hours in the third trimester. This is why ACOG recommends limiting caffeine to <200mg/day during pregnancy.

Smoking: Smoking induces CYP1A2, halving caffeine half-life to ~2-3 hours. Smokers can often drink coffee later in the day without sleep disruption. (But smoking has far worse health effects than caffeine, so this is not a benefit.)

Age: Newborns have very long caffeine half-lives (80+ hours). Children have shorter half-lives than adults. Elderly adults have slightly longer half-lives.

Liver disease: Cirrhosis and hepatitis impair caffeine metabolism, extending half-life.

Medications: Fluvoxamine, ciprofloxacin, and mexiletine inhibit CYP1A2. Carbamazepine and rifampin induce it.

How much caffeine disrupts sleep?

Research suggests that caffeine doses as low as 50mg (half a cup of coffee) consumed 3 hours before bed can delay sleep onset by 10-20 minutes and reduce deep (slow-wave) sleep by 10-20%. Doses of 200mg (one strong coffee) consumed 6 hours before bed still significantly disrupt sleep, this is why the general rule is to stop caffeine 8+ hours before bedtime. If your half-life is 8 hours (slow metabolizer), you need to stop even earlier, 10-12 hours before bed.

Using this calculator

Enter each caffeine dose (coffee, tea, energy drink, pre-workout, chocolate, etc.) with the time consumed. The calculator models caffeine decay using your personalized half-life and shows the total caffeine remaining at your bedtime. If the result is >30mg, consider adjusting your caffeine timing. The calculator assumes single-dose pharmacokinetics, in reality, there are small second-order effects from accumulation, but the single-dose model is accurate enough for practical use.

How to reduce caffeine if it's disrupting sleep

If you're a slow metabolizer or your bedtime caffeine is >50mg: (1) Shift your last coffee earlier, try 10am instead of 2pm. (2) Switch to half-caf or decaf for afternoon coffee. (3) Replace afternoon coffee with green tea (lower caffeine, L-theanine for calm focus). (4) Try a caffeine cutoff experiment, no caffeine after noon for 2 weeks and see if sleep improves. (5) Get a genetic test (23andMe, Nutrition Genome) if you want to confirm your CYP1A2 status. (6) Be especially careful on oral contraceptives or during pregnancy, your half-life is doubled.