What Is Reaction Time? A Clear Explanation

Reaction time is the time between a stimulus appearing (a light, a sound, a touch) and you making a deliberate response to it, such as pressing a button. It is usually measured in milliseconds (ms); one millisecond is a thousandth of a second.

What happens during a reaction

Even the simplest reaction is a chain of events, and every link takes time:

1. Detection: your eye or ear registers the stimulus and converts it into nerve signals.
2. Processing: your brain recognises that the signal has occurred and decides to respond.
3. Motor command: a command travels from the motor cortex down to the muscles of your hand.
4. Movement: the muscles contract and your finger presses the button.

In an online test, two more steps are added: the time your screen or speakers take to present the signal, and the time your mouse, keyboard or touchscreen take to report your press.

The main types of reaction time

Choice reactions are slower than simple reactions because you have to identify the stimulus and pick the right response. Hick's law describes how this decision time grows as the number of equally likely options increases.

Reaction time vs. reflexes

A true reflex, like the knee-jerk, is an involuntary response handled largely by the spinal cord, without a conscious decision. Reaction time tests measure voluntary responses. In everyday language people say “reflexes” to mean fast reactions, which is what our reflex test measures.

How reaction time is measured

Researchers present a stimulus after an unpredictable delay and record the time until a response, repeating it many times. Randomising the delay matters: if the timing were predictable you could anticipate it rather than react. Responses that are impossibly fast (commonly under 100 ms) are treated as anticipations.

Why reaction time varies

Your reaction time is not a fixed number. It shifts with alertness, fatigue, sleep, age, practice and even how much you anticipate the signal. That is why good tests use several attempts and report the median and spread as well as the average. You can read more in our guide to what counts as a good reaction time.