Quick Answer: There's no universal cutoff. For one average cup and an average metaboliser, caffeine takes roughly five hours to fall below 50 mg; a bigger cup, a slower metabolism, or an earlier coffee still in your system pushes that earlier. Use our free Caffeine Half-Life Calculator to work out your cutoff from your bedtime and cup strength.
Caffeine is a powerful tool for focus and performance, but its timing can make or break your sleep. Understanding how your body clears it is the key to keeping the benefit without paying for it at night.
The Half-Life Rule#
Caffeine's half-life is roughly 4 to 7 hours depending on the person (around 5 on average). This means that if you drink a cup of coffee at 4 PM, about half of that caffeine is still circulating around 9 or 10 PM. Even if you are someone who can fall asleep after coffee, caffeine still in your system can reduce deep sleep. In one study, 400 mg taken 6 hours before bed still cut sleep by about an hour (Drake et al., 2013, Journal of Clinical Sleep Medicine).
For one standard cup (about 95 mg), here is roughly how long it takes to fall below 50 mg:
- Fast metaboliser (half-life about 4 hours): about 3.5 to 4 hours
- Average metaboliser (about 5.5 hours): about 5 hours
- Slow metaboliser (about 7 hours): about 6.5 hours
ARC does this math for every drink you log and shows the result as one time: your last safe coffee.
How ARC works out your cutoff#
There's no single buffer. ARC computes your last safe coffee from your bedtime, your caffeine sensitivity (half-life about 4, 5.5 or 7 hours), everything you've already drunk today and a realistic next drink, aiming for under 50 mg still in your system at bedtime. For one standard cup and an average metaboliser that's roughly 5 hours before bed; two cups (~190 mg) push it to about 10–11 hours.
If you plan to be in bed by 10 PM, one cup by about 5 PM leaves an average metaboliser at roughly 50 mg by bedtime; a double or a second cup moves that back to around noon. If you'd rather not do the math, many people find mid-afternoon a sensible default. For the full model, see the caffeine half-life guide or how much of a 3 PM coffee is left at bedtime.
The Adenosine Block#
Caffeine works by resembling adenosine, the molecule that builds up in your brain the longer you are awake. By sitting in the adenosine receptors, caffeine keeps you from feeling tired. When the caffeine fades, you feel the adenosine that built up in the meantime. This is what we call the "caffeine crash" (more in why coffee makes you tired). By timing your coffee well, you can soften this crash and make sure that you are naturally tired when it's actually time to rest.