Sketchplanations
The half-life of caffeine illustration: showing the effect of caffeine rising then decreasing over time

The half-life of caffeine

The half-life of caffeine is about 4 hours.

The data vary, but for a medium-sized adult, you can probably expect half of the caffeine from that 3 pm cup of coffee to still be doing half of its thing about 7 pm, 4 hours later. Apparently, smokers deal with caffeine quicker, and caffeine sticks around in children much longer.

You can expect about 10 minutes before it first kicks in, which, I suppose, is why if you’re feeling tired while driving, it’s a decent plan to take a break and drink a coffee, then have a 15-minute nap. See tiredness can kill, take a break. Once it starts to kick in, you might hit peak caffeine any time from 45 minutes to 2 hours later.

Caveat: The chart visualisation is extrapolated from reported data, so the shape is only approximate.

Some references say longer, and some say from 3-9 hours or so. My source, which seemed pretty comprehensive, was:

Bertil B. Fredholm, Karl Bättig, Janet Holmén, Astrid Nehlig and Edwin E. Zvartau, Actions of Caffeine in the Brain with Special Reference to Factors That Contribute to Its Widespread Use, Pharmacological Reviews March 1999, 51 (1) 83-133;

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