Reaction test: what really happens in those 200 milliseconds

Published September 20, 2026 · ~4 min read

The screen turns green, you tap, and a number stops somewhere between 200 and 300 milliseconds. It is the finish line of a fairly long relay.

Three legs in 200 ms

The first leg is sensing. Light hits the retina, the signal reaches the visual cortex, and the brain registers a change. Thirty to fifty milliseconds, much the same for everyone.

The second leg is deciding. The brain has to agree this is not a glitch and worth moving a finger for. This part is all over the place. Staring at the screen with nothing else on your mind, it gets squeezed very short. Replaying an email in your head, it stretches out several times over.

The third leg is moving. The motor cortex fires, the signal runs to the forearm, the muscle contracts. Tens of milliseconds, close to fixed, barely moved by practice.

A reaction test, then, is not measuring how fast your hand is. It measures the least stable leg: attention.

There is a floor on fast

Track and field makes the point. A sprint start inside 100 milliseconds of the gun is a false start. Not to punish eagerness — nobody can genuinely react that fast. Getting the sound into the ear and down to the legs uses up those 100 milliseconds on its own. Beat it and you were guessing the gun, not hearing it. Usain Bolt's reaction in the 2012 Olympic final was 146 milliseconds, which is elite.

On simple reaction alone, the spread between people is not that wide and the ceiling does not move. To get clearly faster you need something other than raw speed.

Anticipation is what separates people

Sports science has a standard trick called temporal occlusion: cut a bowling clip at a chosen frame and ask the viewer where the ball is going. Even when the clip stops the moment the ball leaves the hand, experienced players are far more accurate than beginners. They read it from the arm angle, the shoulder position, the first few centimetres of flight. The beginner waits for the ball to travel; the expert already has the answer.

To a spectator it looks like "he reacts so fast". In motor terms it is usually "he started earlier". Which is why plenty of athletes stay competitive past their physical peak: reaction speed declines with age, reading the play keeps improving.

The more practical use: a status gauge

Labs use something called a psychomotor vigilance task. The procedure is nearly identical to this page, except that it ignores the average and counts the lapses — trials where the time spikes past 500 milliseconds.

Tests like this are unusually sensitive to sleep loss. After a long stretch awake, reactions slow and brief microsleeps appear: your eyes are open, the brain is offline for a second or two. It is why drowsy driving keeps getting compared to drunk driving. So reaction time works as a gauge — time a few rounds in the morning, and if you are thirty or forty milliseconds off your usual, today is not the day for work needing full concentration.

A few honest notes

A browser test always reads higher than a lab: the signal crosses the browser, the display, the screen or keyboard, and each adds tens of milliseconds. Do not compare the number across devices, and do not treat it as medical.

Then the early taps. Tapping before green is a foul here, and not to be annoying — tapping early measures expectation, not reaction. Wait for the colour.

Not much of this is trainable; your state is. Sleep enough, and skip testing in the post-lunch slump. The reaction test on this site runs the same procedure: a few seconds a round, scores staying in your own browser.