Reaction Time: What It Measures and What Affects It

Reaction time sounds like something for racing drivers and gamers, but it is one of the oldest and simplest measures in psychology. It tells you how quickly your brain turns a signal into an action. Because it is so sensitive to your current state, it is also a handy way to see how alert you are on a given day, as long as you know what it can and cannot tell you.

What happens in those milliseconds

Between the moment a light appears and the moment your finger presses a key, several things happen in quick succession.

1. Perception. The eyes register the change and the signal travels to the brain, which recognises it.
2. Decision. The brain determines what the signal means and which response is needed.
3. Movement. A command goes to the muscles, and the finger moves.

In a simple task with one signal and one response, healthy young adults usually react to a visual signal in roughly 200 to 300 milliseconds. Reactions to a sound are often slightly faster. Much of that time is not about the fingers at all; it is about processing.

Simple and choice reactions

Things change when there is more than one possible signal. If a red light means "press left" and a green light means "press right", you first have to work out which one you saw, and the answer takes longer.

In the early 1950s, William Hick and Ray Hyman independently described a regularity that is now known as the Hick–Hyman law: as the number of equally likely options grows, reaction time increases, but not in a straight line. Each doubling of the number of options adds roughly the same amount of time. That is why a familiar, predictable choice feels almost instant, while an unfamiliar one with many options slows you down.

There is also a trade-off between speed and accuracy. You can respond faster if you are willing to make more mistakes, and you can be more accurate if you slow down. Any fair measurement of reaction time should look at errors as well as speed.

What affects your reaction time

• Alertness and sleep. Lack of sleep reliably makes reactions slower and, more importantly, less consistent, with occasional long lapses.
• Fatigue and time on task. After a long, monotonous stretch, responses tend to slow down.
• Expectation. If you know roughly when the signal will come, you react faster. Good tests vary the waiting time so you cannot simply guess.
• Age. In large population studies, average reaction time gradually becomes slower and more variable with age through adulthood. The differences between individuals of the same age are large.
• Practice. Familiarity with a particular task makes you faster at it, especially at first.
• Substances. Alcohol slows reactions; caffeine may slightly speed them up in some situations, especially when you are tired.
• Equipment. Screens and input devices add their own delay, often tens of milliseconds, and it differs between devices.

How to read your own numbers

A single attempt says very little. Reaction time naturally varies from trial to trial, so it is more useful to look at the typical value across a series, such as the median, and at how consistent you are.

Compare yourself with yourself, not with others. Different phones, monitors and keyboards make cross-device comparisons unreliable. If you want to track changes, measure on the same device, at a similar time of day, and in similar conditions.

A common mistake is chasing a single record. Pressing a moment before the signal appears can produce an impressively low number, but that is a guess, not a reaction, which is why good tests count such presses as errors. Your typical time together with your error count is a far more honest picture than your best attempt.

Look at trends rather than single days. One slow session after a bad night is normal. A gradual change over weeks is more informative, and it is always worth considering ordinary explanations first: sleep, stress, workload.

Finally, remember that being quicker in a reaction test makes you quicker in that test. It is a useful indicator and a pleasant challenge, not a measure of intelligence or a guarantee of faster reactions in everyday life.

What to try

The Reaction exercise in Fokusnyk measures exactly this: how quickly you respond to a signal and how many errors you make along the way. Do several rounds in a row, look at your typical time and your errors, and keep notes on how you slept. Over a couple of weeks, you will get a clear picture of how your daily state shows up in your own results.

Sources

• Hick, W. E. (1952). On the rate of gain of information. Quarterly Journal of Experimental Psychology, 4(1), 11–26.
• Hyman, R. (1953). Stimulus information as a determinant of reaction time. Journal of Experimental Psychology, 45(3), 188–196.
• Der, G., & Deary, I. J. (2006). Age and sex differences in reaction time in adulthood: Results from the United Kingdom Health and Lifestyle Survey. Psychology and Aging, 21(1), 62–73.
• Lim, J., & Dinges, D. F. (2010). A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin, 136(3), 375–389.