Neuroscience
2 minutesInteractive tool
Caffeine Calculator
Your 4pm coffee is still awake at midnight.
Tell it what you drank and when, and it tells you how many milligrams of caffeine will still be circulating when you turn off the light — verdict included.
You fall asleep, so you think coffee doesn't affect you. But falling asleep isn't the same as sleeping well: the caffeine left in your blood cuts into deep sleep without asking.
Why it works
Caffeine doesn't vanish when the buzz fades: it clears exponentially, with a half-life of about five and a half hours in a typical adult. Translation: half of your 4pm coffee is still in your blood past nine, and a quarter of it is still there around 3am.