Caffeine and sleep
How Late Can You Drink Coffee Before Bed?
The honest answer is that it depends on your dose, your bedtime and your own clearance rate. Here is the arithmetic, and how an app can turn it into something you can actually check against your sleep.
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The arithmetic
Half-life is why the afternoon coffee still counts
Caffeine's half-life is the time it takes your body to clear half a dose. Sleep researchers commonly put it at roughly five to seven hours in healthy adults, and the individual range quoted in the literature is much wider, often given as two to twelve hours. That single number has an uncomfortable consequence: clearance is not linear, so the tail is long.
Take a 200 mg coffee at 15:00 on a six-hour half-life. At 21:00 you are at 100 mg. At 03:00 you are at 50 mg. You went to bed somewhere in the middle of that, with most of the dose still circulating. That is the whole problem in one line, and it is why "I can fall asleep on coffee" and "coffee is not affecting my sleep" are two different claims.
The limits
What a phone can and cannot know
- It cannot measure your blood. Every caffeine app, Circa included, runs a decay model over what you typed in. If the log is wrong the curve is wrong.
- It does not know your clearance rate. Genetics, notably CYP1A2 variants, medication, pregnancy and smoking all shift it. A generic half-life is an assumption, not a finding about you.
- It can measure the association. This is the part that is real. If you log consistently and your tracker records sleep stages, the relationship between caffeine at bedtime and your deep-sleep minutes is something an app can estimate from your own nights and report with an interval around it.
Circa is a personal tracking tool, not a medical device. It does not diagnose, treat, or prevent any condition, and it gives no medical advice. Its numbers describe your own logs and your own tracker readings, nothing more. Talk to a qualified professional before you change medication, supplementation, or anything else about your health.
What Circa does with it
From a curve to a number you can argue with
Circa draws the decay curve to your bedtime with a 50 mg reference line, and shows two figures above it: caffeine in your system now, and caffeine projected at bedtime. That is the planning half.
The measuring half runs in the Signals tab. Circa fits your caffeine at bedtime against the deep-sleep minutes your tracker recorded, controlling for training load, weekday, alcohol and bedtime, and reports the effect per 100 mg with n, a 95 percent confidence interval and a p-value, plus the scatter plot of the nights behind it. If the interval fails to clear zero for 14 days in a row, the insight retires itself.
Questions
Common questions
How long does caffeine stay in your system?
Sleep researchers commonly put caffeine's half-life at roughly five to seven hours in healthy adults, with a wide individual range that is often quoted as two to twelve hours. Half-life is the time it takes for half the dose to clear, so the tail is long: a 200 mg coffee at 15:00 is still worth roughly 100 mg at 20:00 and roughly 50 mg at 01:00 on a six-hour half-life.
What is the 50 mg line?
Circa draws a horizontal line at 50 mg of caffeine remaining and marks where your bedtime crosses the decay curve. It is a threshold the app uses consistently so you can compare one day against another. It is not a medical limit and Circa does not claim one.
Can an app know how fast I metabolise caffeine?
No, and Circa does not pretend to. The curve is a decay model applied to what you logged, not a measurement of your blood. Clearance varies between people, notably with CYP1A2 genetics, and the model does not know yours. What Circa can do is measure the association in your own nights: it fits caffeine at bedtime against your measured deep-sleep minutes and reports the effect with n, a 95 percent confidence interval and a p-value.
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