Caffeine Half-Life Calculator
Caffeine has a half-life of about five hours. The afternoon coffee is still doing work at midnight. Enter when you drank it and we'll show how much is in your system at bedtime.
28.7% of the original dose, after 9 hours. Enough to fragment sleep architecture even if you fall asleep.
To get below 50mg (the rough sleep-disturbance threshold), you'd need 10 hours from drinking. So with this dose, your latest "safe" cutoff is roughly 13:00 — earlier if you're a slow metaboliser.
How this works
Caffeine is metabolised by the liver enzyme CYP1A2. The half-life — the time for half the dose to be cleared — averages 5 hours in healthy adults but ranges 1.5–9.5 hours depending on genetics, smoking status, hormonal contraceptives, pregnancy, and certain medications.
Caffeine works by blocking adenosine receptors. While it's bound, your brain doesn't feel sleep pressure even though it's accumulating. When the caffeine clears, the backed-up adenosine floods receptors all at once — the "crash". Caffeine also reduces deep sleep and REM, even at doses you don't notice subjectively.