Caffeine Half-Life & Bedtime Calculator
Enter up to three caffeine doses and when you took them. The calculator estimates how much caffeine is still active at bedtime, how much carries into waking, and when an additional dose would need to stop.
Estimated caffeine remaining at bedtime18.95
- Total logged caffeine
- 100
- Share of logged caffeine remaining
- 18.95%
- Bedtime target status
- Below the 50 mg comparison level.
- Time after bedtime to reach target
- 0
- Estimated caffeine at next waking
- 6.25
- Latest cutoff for the additional dose
- 8.44 hours before bedtime.
Dose breakdown at bedtime
| Dose | Logged caffeine | Hours before bed | Bedtime remainder |
|---|---|---|---|
| Dose 1 | 100 | 12 | 18.95 |
| Dose 2 | 0 | 6 | 0 |
| Dose 3 | 0 | 3 | 0 |
Uses first-order elimination and assumes each dose is absorbed immediately.
The bedtime target is a comparison value, not a sleep-safe threshold.
How to use this calculator
- Enter each drink, food, supplement, or medication dose in milligrams.
- For each dose, enter how many hours before your planned bedtime it was taken.
- Choose a half-life assumption and a bedtime comparison target.
- Enter your expected sleep duration to estimate caffeine at waking.
- Use the planned dose field to reverse-solve a latest cutoff time before bed.
How the calculator estimates caffeine at bedtime
Caffeine is commonly modeled with first-order elimination: after one half-life, half of the absorbed caffeine remains; after two half-lives, one quarter remains. The National Academies review describes caffeine elimination as first-order and adequately represented by a one-compartment open model, while also noting that half-life varies widely between people.
For each logged dose, the calculator uses:
B_i = D_i x 2^(-h_i / H)
- D_i is the caffeine dose in milligrams.
- h_i is the number of hours between that dose and bedtime.
- H is the assumed caffeine half-life in hours.
- B_i is that dose's estimated caffeine remaining at bedtime.
The bedtime total is B = sum(B_i). The share remaining is 100 x B / D, where D is the total logged caffeine. If no caffeine is logged, the share is shown as zero. Caffeine at waking after S hours of sleep is W = B x 2^(-S / H).
How the bedtime cutoff is solved
The target maximum is a comparison level you choose. It is not a clinical line between safe and unsafe sleep. If your estimated bedtime caffeine is already above the target, the calculator reports that no additional dose can meet it. Otherwise, it treats the difference as the remaining bedtime allowance.
For a planned additional dose P, the cutoff is:
h_cutoff = max(0, H x log2(P / A))
Here A = L - B, where L is your bedtime target. If the planned dose is smaller than the allowance, the model says it could be taken at bedtime and still remain under the selected target. That is a math result, not a sleep recommendation.
What changes the result most
The two biggest drivers are timing and half-life. A 100 mg dose taken five hours before bed leaves about 50 mg when the half-life is five hours, but about 71 mg when the half-life is nine and a half hours. Several smaller doses can also accumulate because each dose has its own remaining fraction at bedtime.
The FDA cites 400 mg per day as an amount not generally associated with negative effects for most adults, while emphasizing that sensitivity and elimination speed vary. A sleep study in the Journal of Clinical Sleep Medicine found that 400 mg of caffeine taken at bedtime, three hours before bed, or six hours before bed significantly disrupted sleep compared with placebo.
What this calculator leaves out
The estimate assumes instant absorption, a constant half-life, and no interaction between doses. Real absorption can take from minutes to hours, and clearance can shift with pregnancy, genetics, smoking, oral contraceptives, other medications, liver function, age, and individual sensitivity. The calculator estimates remaining caffeine mass from entered doses; it does not measure blood caffeine concentration, alertness, sleep quality, or personal medical risk.
Worked example
Suppose you had 100 mg of caffeine 12 hours before bed, 80 mg six hours before bed, and 40 mg three hours before bed. With a five-hour half-life, the bedtime remainders are about 18.95 mg, 34.82 mg, and 26.39 mg, for a total of 80.16 mg at bedtime.
The total logged caffeine is 220 mg, so about 36.44% remains at bedtime. If your comparison target is 50 mg, the estimate is above target and would take about 3.4 more hours after bedtime to fall to that level. After eight hours of sleep, the model estimates about 26.44 mg remaining at waking. Because the bedtime estimate is already above the target, no additional 100 mg dose can meet that selected target.
Common questions
What is caffeine's average half-life?
The National Academies review gives about five hours as a mean plasma half-life in healthy individuals, with a reported range from about 1.5 to 9.5 hours. That range is why this calculator lets you choose the half-life instead of hard-coding one value.
How much caffeine is still in my system at bedtime?
It depends on the dose, the time since the dose, and your half-life assumption. For example, 100 mg taken five hours before bedtime leaves about 50 mg if the half-life is five hours, before accounting for absorption and individual variation.
Can caffeine from several drinks accumulate?
Yes. Each dose keeps decaying after you take it, but later doses have less time to clear before bed. The calculator estimates each dose separately and then adds the bedtime remainders.
When should I stop drinking coffee before bed?
There is no single cutoff that fits everyone. The sleep study cited here found measurable disruption from 400 mg even six hours before bedtime, and the math shows longer half-life assumptions push the cutoff earlier.
Is 50 mg of caffeine at bedtime safe for sleep?
No universal sleep-safe threshold is established by this calculator. The 50 mg default is only a comparison value that makes the result easier to interpret; sensitive people may notice effects below it, while others may respond differently.
Does this calculator measure blood caffeine concentration?
No. It estimates remaining caffeine mass from the doses you enter using a simplified elimination model. Blood concentration depends on absorption, body size, distribution, metabolism, and measurement timing.