Last updated: September 24, 2026
What the bedtime number means
The bedtime estimate answers one question: if every drink you logged decays at the same rate, how much caffeine is expected to still be in your system at the hour you plan to sleep? It is a single number that folds a whole day of drinks into one figure you can compare against the 25 mg bedtime planning target.
It is deliberately narrow. The estimate says nothing about how quickly you fell asleep, how well you slept, or how you felt the next morning. It describes an amount, using one stated assumption.
Why each drink gets its own clock
Caffeine does not clear in a straight line, and it does not clear as a single daily total. Each dose decays from the moment you finish it, so a drink taken earlier has already lost more of itself by bedtime.
A 95 mg coffee drunk at 09:00 contributes roughly 14 mg to a 23:00 bedtime, while the same 95 mg coffee drunk at 14:00 still contributes about 27 mg to the same bedtime. Identical drinks, nearly double the contribution, purely because of timing. Summing those individual remainders is what the calculator does for every entry in the list.
Worked examples
Every row uses a five-hour half-life and reports the estimated total remaining at the bedtime shown.
| Drinks logged | Bedtime | Estimated remaining |
|---|---|---|
| 95 mg coffee at 09:00 | 23:00 | 14 mg |
| 95 mg coffee at 09:00 and 14:00 | 23:00 | 41 mg |
| 95 mg at 09:00, 12:00 and 15:00 | 23:00 | 66 mg |
| The same three coffees | 01:30 | 46 mg |
| 150 mg pre-workout at 18:00 | 23:00 | 75 mg |
| 63 mg espresso at 17:00 | 23:00 | 27 mg |
| 28 mg green tea at 16:00 | 23:00 | 11 mg |
| 95 mg at 07:00, 10:00, 13:00 and 16:00 | 23:00 | 86 mg |
Amounts are rounded. The pattern to notice is that a single well-timed drink stays small, while several drinks spread across the day stack up even when none of them looks large on its own.
Bedtime matters as much as the drinks
The two middle rows of the table above use exactly the same three coffees and differ only in when the person goes to sleep. Reading 66 mg at 23:00 and about 46 mg at 01:30 is not a contradiction; it is the same day measured at two different hours.
This is why a late sleeper can log a fairly heavy day and still see a moderate number. It is also why the estimate should be read together with what you actually drank. A low bedtime figure after six coffees describes the hour you chose, not a light day of intake.
What the number does not tell you
- It is not a sleep prediction. Sleep depends on far more than caffeine, and the estimate cannot see any of it.
- It is not a sensitivity rating. Two people with the same number may feel completely different, because sensitivity and tolerance sit outside the model.
- It is not a personal half-life. The same five-hour assumption is applied to everyone, and several factors can move the real value in either direction.
- It is not advice. The output is a planning estimate, and the decision about a later coffee stays with you.
How to use it
The most useful habit is to enter drinks as you go and check the estimate once. If the number at bedtime is higher than you expected, the two levers available are the size of the next drink and the hour you plan to sleep, and the cutoff estimate translates those into a single clock time.
If you are unsure what to enter for a drink, the caffeine content chart lists representative amounts for common sources.
Common questions
Is the bedtime number a measurement?
No. It is a model estimate built from the doses you entered, the times you entered, and one stated five-hour half-life. It is useful for comparison and planning, and it is not a reading of your blood or of your sleep.
Why do three coffees not simply triple the amount?
Because each drink starts its own clock when you finish it. An earlier coffee has had more time to decay by bedtime, so it contributes less than a recent one. The estimate decays each drink separately and then adds the remainders.
Does a late bedtime cancel out the caffeine?
It lowers the estimate, but it does not cancel the intake. Sleeping later gives every drink more time to decay, which is why the same three coffees can read 66 mg at 23:00 and about 46 mg at 01:30. What you consumed is unchanged; only what remains at that hour moves.
Could I feel more than the estimate suggests?
Possibly. The number describes an amount, not an effect. Sensitivity, tolerance, how fast you absorb caffeine, and how you sleep in general all sit outside the calculation, so treat the estimate as one input rather than a verdict.
Related reading
- Caffeine Half-Life: How Long Caffeine Stays in Your System
- Caffeine Metabolism: How Your Body Clears Caffeine
- Caffeine Content Chart for Common Drinks
- Caffeine Cutoff Time: When Should You Stop Drinking Coffee?
- Coffee Before Bed: How Late Is Too Late to Drink Coffee?
- What Changes Caffeine Half-Life?
- Am I a Slow Caffeine Metabolizer? A Self-Check
- How Much Caffeine Is Left? Dose by Hour Chart
Run your own numbers
The CaffCutoff calculator turns these ideas into an estimate for your own day: add each drink, set a target bedtime, and read the remaining caffeine, the cutoff estimate, and the decay timeline.