What Does Caffeine Do in the Brain?
Why does a cup of coffee wake you up? The key molecule is adenosine. Adenosine builds up in the brain while you're awake and sends a "time to rest" signal — it's essentially a sleepiness-inducing molecule. Caffeine blocks adenosine from binding to its receptors. It gets to the receptors first and occupies them, so the sleepiness signal is transmitted less strongly, and your arousal level rises.
This action is directly tied to reaction time. When arousal is low, the whole chain — from detecting a signal to producing a response — slows down. Caffeine raises arousal, and that chain runs a little faster. It's not that you "feel better so you react faster"; the background state of neural transmission actually changes.
How Many Milliseconds Does It Save?
This is the question everyone wants answered. The short answer: it varies by study, but improvements in the tens of milliseconds are commonly reported. In typical simple reaction-time tasks, intake of around 200mg (roughly 1–2 cups of coffee) has been associated with reaction-time reductions in the 20–50ms range across several studies. Of course, depending on baseline reaction time, the measurement method, and participant state, results outside this range also exist. Some studies report larger gains; others find almost no difference.
What matters is not "caffeine always shaves off 50ms" but that on average there is a significant improvement, and the size varies a lot between individuals. Concluding "my caffeine effect is 30ms" from a single measurement is premature.
When Does the Effect Peak?
Caffeine doesn't act the instant you drink it. It has to be absorbed from the gut, and blood concentration takes time to rise. Generally, blood concentration peaks 30–60 minutes after intake, and that's when the arousal effect is greatest. The half-life is roughly 3–5 hours — after that, concentration drops by half and the effect gradually fades.
So "drinking coffee 10 minutes before a test" means you're measuring in a state where the effect has barely kicked in. To see the effect properly, it's reasonable to wait about 45 minutes after intake before measuring.
Daily Drinkers Get a Smaller Effect
Caffeine comes with tolerance. People who drink coffee every day develop changes in adenosine-receptor sensitivity and expression, so their baseline arousal is already adjusted to caffeine. In other words, the "jump" a non-drinker gets from one cup is much smaller for a daily drinker.
This is essential when designing a measurement. Before concluding "coffee doesn't speed up my reaction time," ask yourself whether you're a daily drinker or not. Conversely, someone who rarely drinks caffeine may see a relatively large change after a single cup.
Too Much Backfires
More caffeine isn't always better. At high doses, side effects like anxiety, tremors, and elevated heart rate appear. In reaction-time tests this often shows up as early taps — your hand moves before the signal has actually appeared. In the Reaction Time game, this looks like an increase in taps registered before the green light, which get counted as invalid.
In other words, caffeine making reactions "faster" and making them "premature" are different things. A fast reaction is quick execution after accurately detecting a signal; a premature reaction is an uninhibited, hasty move. Push the dose too high and the latter increases, so overall accuracy and valid reaction time can actually get worse.
A Hands-On Experiment Idea
The most reliable way to see what caffeine does for you is to measure it yourself. Try a simple comparison experiment.
| Condition | Method | |-----------|--------| | Baseline | At least 6 hours after any caffeine, in the morning, take 5 Reaction Time trials | | After caffeine | Same day, drink 1 cup of coffee (~100mg), wait 45 min, take 5 trials | | High dose | On another day, drink 2–3 cups, wait 45 min, take 5 trials and also record pre-green-light taps |
This lets you roughly check (1) how much caffeine reduces your reaction time, and (2) whether raising the dose improves things further or just increases premature taps. Since many variables — sleep, time of day, meals — aren't controlled, don't judge from a single session; repeat over a few days.
This is an area with large individual differences, so the values you get are reference data meaningful only for you. Keep in mind this is not medical advice or a diagnosis.