حاسبة نصف عمر الكافيين

Caffeine is cleared from the body on a predictable exponential curve, which is what makes a half-life calculator useful: it turns "I had a coffee at 3pm" into an estimate of how much is still circulating at bedtime. The average half-life in a healthy adult is about five hours, but that average conceals a range from roughly two to ten hours driven mostly by one liver enzyme, which is why the same afternoon coffee wrecks one person sleep and not another.

المعادلة

Remaining (mg) = Dose x (0.5 ^ (hours elapsed / half-life))
Default half-life = 5 hours

• Dose = caffeine consumed in milligrams
• hours elapsed = time since consumption
• half-life = hours for the amount in the body to halve, around 5 in a healthy non-smoking adult

First-order elimination kinetics. Caffeine is metabolised primarily by the hepatic enzyme CYP1A2, which accounts for over 90 percent of its clearance.

How the calculation works

Caffeine follows first-order kinetics, meaning a constant fraction is removed per unit time rather than a constant amount. Each half-life removes half of what remains, so 200 mg at noon becomes 100 mg at 5pm, 50 mg at 10pm and 25 mg at 3am on a five-hour half-life. That trailing quarter is the reason people who sleep poorly often underestimate how long their coffee is with them.

The practical implication is that the last dose of the day matters more than the total. Peak blood concentration occurs 30 to 60 minutes after drinking, so the timing question is not when you feel the effect but how much remains at the hour you want to fall asleep.

Why your half-life may not be five hours

CYP1A2 activity varies widely, partly through a common genetic variant that divides people into fast and slow metabolisers, and partly through things that induce or inhibit the enzyme. The differences are large enough to matter in daily life, not just in pharmacology.

• Tolerance and half-life are different things. Habitual drinkers feel less effect from the same blood level, but they do not clear it faster.
• A smoker who quits does not change their caffeine dose but does roughly double its persistence, which is a common and unrecognised cause of insomnia and jitteriness after quitting.

Caffeine and sleep

A controlled trial giving 400 mg of caffeine at 0, 3 and 6 hours before bed found measurable disruption of sleep even in the 6-hour condition, with participants losing more than an hour of sleep, and importantly, not reliably noticing it. Subjective sleep quality was a poor guide to what the sleep recordings showed.

That is the strongest practical argument for using a half-life estimate rather than intuition. If a 2pm coffee still leaves meaningful caffeine on board at 10pm, and people are poor at detecting the resulting sleep disruption, then a cut-off time set from the arithmetic is more reliable than one set from how alert you feel.

• The FDA cites 400 mg a day as an amount not generally associated with negative effects in healthy adults, roughly four or five cups of coffee.
• Pregnancy guidance is substantially lower, commonly 200 mg a day, and the prolonged half-life in late pregnancy is part of the reason.

حدود الاستخدام

• The default five-hour half-life is a population average. Individual values range from about two to ten hours in healthy adults, and much longer in pregnancy, liver disease or with interacting medications.
• The calculator cannot know your CYP1A2 activity, your smoking status or your medications, all of which change the answer substantially.
• Caffeine content in real drinks varies enormously. Brewed coffee ranges from roughly 70 to 200 mg per cup depending on bean, grind and method, so the dose entered is itself an estimate.
• The model describes the amount remaining in the body, not the effect felt, which is modified by tolerance, adenosine receptor density and time of day.
• It does not apply to newborns, in whom caffeine clearance is measured in days rather than hours, and who are dosed therapeutically under medical supervision.
• This is an educational estimate, not medical or pharmacological advice.

الأسئلة الشائعة

How long does caffeine stay in your system?

With an average half-life of about five hours, a 200 mg dose leaves roughly 100 mg after 5 hours, 50 mg after 10 and 25 mg after 15. Practically, meaningful amounts persist for 10 to 12 hours in most people, and considerably longer in slow metabolisers.

What is the half-life of caffeine?

About five hours on average in a healthy non-smoking adult, but the normal range spans roughly two to ten hours. Smoking roughly halves it, oestrogen-containing contraceptives roughly double it, and late pregnancy can extend it beyond 15 hours.

When should I stop drinking coffee to sleep well?

A controlled trial found that 400 mg taken even six hours before bed measurably disrupted sleep, and participants did not reliably notice. Working back from your bedtime using the half-life is more reliable than judging by how alert you feel.

Does drinking more coffee make me clear it faster?

No. Regular intake builds tolerance to the felt effect but does not speed elimination. The blood level follows the same curve; you simply notice it less.

How much caffeine per day is safe?

The FDA cites 400 mg a day as not generally associated with negative effects in healthy adults, roughly four to five cups of coffee. Pregnancy guidance is commonly 200 mg, and people with arrhythmias, anxiety disorders or on interacting medications may need less.

المراجع

1. US Food and Drug Administration. Spilling the Beans: How Much Caffeine is Too Much?
2. Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 2013.
3. MedlinePlus. Caffeine.
4. MedlinePlus. Caffeine in the diet.