Caffeine Half-Life Tracker
Project caffeine in your system over time from a list of doses.
Overview
Caffeine clears from the body on a predictable curve, and knowing where you sit on that curve helps explain afternoon jitters, restless sleep, and the diminishing returns of a fourth coffee. A caffeine half-life tracker projects the amount of caffeine remaining in your system over time from one or more doses, so you can plan your last cup of the day before bedtime.
This is especially useful for sleep hygiene: caffeine consumed at 3 p.m. may still leave 80 to 100 mg circulating at 11 p.m., enough to delay sleep onset for many adults. By stacking doses on a timeline, the projection makes the long tail of stimulation visible, which is often more persuasive than vague "avoid coffee after lunch" advice.
How it works
Caffeine follows first-order pharmacokinetics with an average half-life of about five hours in healthy adults. The remaining amount after t hours is dose × 0.5 ^ (t / half_life). Each dose is decayed independently and then summed, so two cups taken three hours apart produce a curve with two staggered exponential drops.
Half-life shifts with biology and lifestyle: pregnancy can stretch it to 10 hours or more, oral contraceptives lengthen it, smoking shortens it to roughly three hours, and genetics in the CYP1A2 liver enzyme account for a large slice of person-to-person variability. The tool exposes a configurable half-life so you can model your own metabolism rather than assume the population average.
Examples
- A 200 mg coffee at 8 a.m. with a five-hour half-life: 100 mg at 1 p.m., 50 mg at 6 p.m., 25 mg at 11 p.m.
- Two 100 mg drinks at 9 a.m. and noon: roughly 130 mg combined at 2 p.m., still 60 mg by 8 p.m.
- A 160 mg pre-workout dose at 5 p.m.: about 80 mg at 10 p.m. — enough to delay sleep for many people.
- An 80 mg green tea at 4 p.m. for a fast metaboliser with a three-hour half-life: only 10 mg remains by 11 p.m.
FAQ
What is a safe daily caffeine limit?
Most healthy adults tolerate up to 400 mg per day, though sensitivity varies and pregnant individuals should consult guidance specific to them.
Why do I sleep poorly even when I "feel fine" by bedtime?
Subjective alertness fades faster than the molecule itself. Residual caffeine still blocks adenosine receptors and reduces deep-sleep duration.
Does decaf have caffeine?
Yes, typically 2 to 15 mg per cup. Not zero, but usually negligible.
Can I shorten the half-life?
Hydration and exercise have only minor effects. The dominant factors are genetics, smoking status, pregnancy, and certain medications.