What the effect looks like
Caffeine is one of the few substances with consistent evidence for improving reaction performance, and the improvement is modest — typically a small number of milliseconds on simple reaction tasks. The larger and more reliable effect is on sustained attention: fewer lapses over a long task, which shows up in your test as a tighter spread rather than a faster average.
The effect is biggest in people who are tired or sleep-deprived, and smallest in people who are already rested and alert. If you sleep well and test in the morning, expect very little. If you test at the end of a long day, expect more.
Timing matters more than dose
- Caffeine peaks in the blood roughly 30 to 60 minutes after drinking it. Testing immediately after a coffee measures the placebo, not the caffeine.
- Its half-life is around five hours, so an afternoon coffee is still working at bedtime and is quietly costing you tomorrow's score.
- Habitual drinkers get less of a boost and more of a withdrawal penalty. Much of a regular drinker's morning improvement is returning to their own baseline.
- Past a moderate dose the returns stop and the jitter starts, which makes fine motor control worse rather than better.
The most common caffeine mistake in testing: drinking more to fix a score that is low because of poor sleep. Caffeine masks sleep loss; it does not replace it.
Test it on yourself properly
This is one of the few things you can actually measure at home with reasonable rigour. Test five trials, caffeine-free, at the same time on three consecutive days to get a stable baseline and a sense of your day-to-day noise. Then test 45 minutes after your usual dose, same device, same time of day, three more days.
Compare the averages, and treat any difference under 15 ms as noise. Then compare the spreads — that is where you are most likely to see the real effect.
What caffeine does not do
It does not make your nerves conduct faster, it does not improve decision quality, and it does not compensate for a missed night of sleep beyond masking how impaired you feel. There is good evidence that sleep-deprived people on caffeine feel substantially more capable than they measure, which is exactly the wrong combination behind the wheel.