Caffeine leaves your system on a predictable decay curve. See how much is left at different times, and roughly when it'll be low enough to not disrupt sleep.
Remaining caffeine = dose × 0.5^(hours elapsed ÷ half-life). This calculator solves that equation backward to find when the remaining amount drops to your chosen threshold.
Caffeine half-life varies significantly by individual — genetics, liver function, pregnancy, smoking, and certain medications can roughly double or halve it. The default of 5 hours is a commonly cited population average, not a personal measurement. This is not medical advice.