Hong Life CalculatorsUnderstanding the caffeine dose and sleep cut-off calculator — concept and principle
Caffeine wakes you up because it lasts for hours, and that same persistence can spoil your sleep. This calculator estimates how much of a dose is still in your body at bedtime, gives the dose range used to improve sports performance and shows the generally accepted daily limit for healthy adults.
How it is calculated
- Left at bedtime (mg) = dose (mg) × 0.5^(hours until bedtime ÷ half-life)
- Performance dose (mg) = body weight × 3 to body weight × 6
- Safe daily limit for adults = 400 mg
The half-life is the time it takes for the body to clear half of the caffeine; the default of 5 hours is a typical adult value.
The principle
Caffeine is absorbed almost completely within about an hour and is then broken down by the liver, mainly by the enzyme CYP1A2. Elimination follows first-order kinetics: in each half-life the amount falls by half, whatever amount you started with. After one half-life 50% remains, after two 25%, and so on, which is why the formula raises 0.5 to the number of half-lives elapsed. The half-life varies widely between people, commonly about 3 to 7 hours. Smoking shortens it, while pregnancy, oral contraceptives and liver disease lengthen it.
Worked example
- Default inputs: 75 kg, 200 mg caffeine, half-life 5 h, 8 h until bed.
- Half-lives elapsed = 8 ÷ 5 = 1.6.
- Remaining = 200 × 0.5^1.6 = 200 × 0.330 ≈ 65.98, shown as 66 mg.
- Performance dose = 75 × 3 to 75 × 6 = 225 – 450 mg.
The calculator shows 66 mg left at bedtime, 225 – 450 mg as the performance range and 400 mg as the daily limit.
| Hours until bedtime | Left at bedtime (mg) |
|---|---|
| 2 | 152 |
| 4 | 115 |
| 6 | 87 |
| 8 | 66 |
| 10 | 50 |
| 12 | 38 |
| 16 | 22 |
How to read the result
66 mg at bedtime is roughly the caffeine of a small cup of coffee still in your system, enough to make sleep lighter or shorter for many people. If sleep matters, move the last dose earlier or take less. With a 3-hour half-life the same dose would leave about 31 mg; with 7 hours, about 91 mg. For sport, 3–6 mg/kg taken about an hour before exercise is the range shown to improve endurance and power; higher doses add side effects rather than benefit.
Common mistakes
- Forgetting tea, cola, energy drinks, chocolate and pain relievers that contain caffeine.
- Assuming the 400 mg limit applies to adolescents or during pregnancy; EFSA advises no more than 200 mg a day in pregnancy.
- Using the performance dose as a daily habit; for many people it already exceeds 400 mg.
Frequently asked questions
How much caffeine is in a cup of coffee?
It varies widely with the bean and brewing method; a typical cup of brewed coffee often contains roughly 80–120 mg.
Does decaf contain caffeine?
A little, usually only a few milligrams per cup.
Sources: EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the safety of caffeine. EFSA Journal, 2015. · Guest NS et al. International Society of Sports Nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021.