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See exactly how caffeine moves through your body and whether that afternoon coffee is sabotaging your sleep. Enter your drink, the time you had it, and your bedtime to get a full breakdown.
Caffeine has a half-life of approximately 5 hours in the average adult. That means if you consume 200mg of caffeine at 8:00 AM, you'll still have roughly 100mg in your system at 1:00 PM, 50mg at 6:00 PM, and 25mg at 11:00 PM. This calculator uses the exponential decay formula: remaining = dose × 0.5^(hours / 5).
Throughout the day your brain accumulates a chemical called adenosine. As adenosine builds up and binds to its receptors, you feel progressively sleepier. Caffeine works by blocking those adenosine receptors — it doesn't eliminate the tiredness, it masks it. When the caffeine wears off, the built-up adenosine floods back onto those receptors, which is why you sometimes feel more tired after a caffeine crash than you would have without it.
You may not feel it: Research shows that caffeine consumed even 6 hours before bed can reduce total sleep time by over an hour — even when subjects reported no perceived difficulty falling asleep. Caffeine doesn't just delay sleep onset; it reduces the amount of deep, restorative slow-wave sleep you get.
The vicious cycle: Poor sleep from late caffeine leads to more tiredness the next day, which leads to more caffeine, which leads to worse sleep again. Breaking this cycle usually means setting a hard cutoff — most sleep researchers recommend no caffeine after early afternoon.
The 5-hour half-life used here is an average. Your actual caffeine metabolism can range from 3 to 7 hours depending on genetics (CYP1A2 gene variants), liver function, age, pregnancy (half-life can double), hormonal contraceptives, and certain medications. Smokers tend to metabolise caffeine faster, while pregnant individuals metabolise it significantly slower. If you find caffeine affects you more or less than average, your personal half-life likely differs from the standard figure.
The Forge helps you log daily habits, monitor recovery, and work with a coach to build a routine that actually sticks — caffeine timing included.
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