The Science of Reaction Time
Reaction time — the interval between a stimulus and your response — is one of the most studied measures in experimental psychology, dating back to the 1850s. Understanding the science behind it reveals how your nervous system works, why some people react faster than others, and how you can sharpen your own reflexes.
What Is Reaction Time?
Reaction time (RT) is defined as the elapsed time between the onset of a stimulus and the initiation of a motor response. It is measured in milliseconds (ms) and represents the speed at which your nervous system detects, processes, and acts on incoming information. A typical human visual reaction time falls between 200 and 250 milliseconds — roughly a quarter of a second.
It is important to distinguish reaction from reflex. A reflex is an involuntary, automatic response that bypasses conscious brain processing — such as pulling your hand from a hot surface. A reaction, by contrast, is a voluntary response that requires cognitive processing: you perceive the stimulus, decide to act, and then execute the movement. Reaction time testing measures this voluntary pathway.
Reaction time is a key metric in cognitive psychology, neuroscience, sports science, human factors engineering, and clinical diagnosis. It serves as a window into the efficiency of your central nervous system. You can test your own reaction time to see where you stand.
The Neural Pathway
When you respond to a stimulus, your nervous system completes a multi-stage relay. Each stage takes measurable time, and the total is your reaction time. Here is the pathway for a visual stimulus:
Stimulus
Light, sound, touch
Sensory Receptors
~30 ms
Afferent Nerves
~20 ms
Brain Processing
~100 ms
Efferent Nerves
~20 ms
Motor Response
~50 ms
Total Visual Reaction Time (typical)
~220 ms
Sensory Transduction (~30 ms)
Specialized receptor cells convert physical energy into electrical nerve impulses. In vision, photoreceptors in the retina respond to light; in hearing, hair cells in the cochlea respond to vibrations. This electrochemical conversion is the first bottleneck in the reaction chain.
Neural Transmission (~20 ms each way)
Afferent (sensory) nerves carry signals from receptors to the brain at speeds of 70-120 m/s. After processing, efferent (motor) nerves carry the response command back to the muscles. Myelinated nerve fibers conduct signals faster than unmyelinated ones, which is why some pathways are quicker than others.
Brain Processing (~100 ms)
The largest component of reaction time. The brain must identify the stimulus, determine its relevance, select the appropriate response, and plan the motor action. This stage involves the sensory cortex, prefrontal cortex, premotor cortex, and basal ganglia. More complex decisions require more processing time.
Motor Execution (~50 ms)
The primary motor cortex sends signals through the spinal cord to activate muscle fibers. The muscle contracts, producing movement — such as pressing a button or clicking a mouse. The speed of this stage depends on muscle fiber type, physical fitness, and the distance the signal must travel.
Types of Reaction Time
Psychologists classify reaction time into three categories, each demanding progressively more cognitive processing. This classification was first proposed by Dutch physiologist Franciscus Donders in the 1860s and remains foundational in experimental psychology.
Simple Reaction Time
One stimulus, one response. You see a light turn green and press a button as fast as possible. This is the most basic form of reaction time, requiring detection but no decision-making. Typical simple RT for visual stimuli is 200–250 ms in young adults. It represents the baseline speed of your nervous system.
Test your simple RT200-250ms
Choice Reaction Time
Multiple stimuli, each requiring a different response. For example, press the left key for a red light, the right key for a blue light. Choice RT adds the cognitive load of stimulus discrimination and response selection. Typical choice RT is 300–400 ms, increasing with the number of alternatives.
Test your choice RT300-400ms
Recognition Reaction Time
Multiple stimuli appear, but you respond only to a specific target and ignore the rest. This is sometimes called "go/no-go" reaction time. It requires both stimulus identification and inhibitory control. Typical recognition RT is 250–350 ms. This type of reaction is common in real-world tasks like driving: you must react to a red brake light but ignore other visual noise.
Test your recognition RT250-350ms
Hick's Law (Hick-Hyman Law)
In 1952, psychologists William Edmund Hick and Ray Hyman demonstrated that reaction time increases logarithmically with the number of stimulus-response alternatives. Formally:
RT = a + b · log2(n)
where n = number of choices, a = base RT, b = slope constant
This means doubling the number of choices adds a roughly constant amount of reaction time (about 150 ms per doubling). Hick's Law has profound implications for user interface design, sports strategy, and military decision-making: reducing the number of options someone must process directly speeds their response.
Factors That Affect Reaction Time
Your reaction time is not a fixed number. It fluctuates based on a wide range of biological, psychological, and environmental factors. Understanding these influences is the first step toward improving your reaction time.
Age
Reaction time improves through childhood as the nervous system matures, reaching peak speed in the early 20s. From the mid-20s onward, RT gradually slows — approximately 1–2 ms per year. By age 60, average reaction times are 20–30% slower than at age 20, due to reduced nerve conduction speed and declining neurotransmitter levels. See average reaction times by age for detailed data.
Stimulus Type
The type of stimulus significantly influences reaction speed. Auditory stimuli produce the fastest reactions (~140 ms), followed by tactile (~155 ms) and visual (~180 ms). The auditory advantage exists because sound processing involves fewer synaptic relays, and the brainstem can trigger motor responses before full cortical analysis. You can compare this yourself with our audio reaction test.
Arousal & Attention
The Yerkes-Dodson law describes an inverted-U relationship between arousal and performance. Moderate arousal produces the fastest reaction times; too little arousal (boredom, drowsiness) or too much (anxiety, panic) both slow responses. Focused attention on the expected stimulus location or modality also accelerates RT by 20–50 ms compared to divided attention conditions.
Fatigue & Sleep Deprivation
Sleep deprivation is one of the most potent impairments to reaction time. Research shows that after 24 hours without sleep, reaction time degrades by an average of 300% and variability increases sharply. Even mild sleep restriction (6 hours vs 8 hours) measurably slows responses. Fatigue reduces neural processing efficiency and increases lapses of attention.
Substances
Caffeine in moderate doses (100–200 mg) can improve reaction time by 5–10% by blocking adenosine receptors and increasing alertness. Alcohol, conversely, slows reaction time at even low blood concentrations (0.02–0.05 BAC), primarily by impairing neural signal transmission in the cerebellum and prefrontal cortex. Other substances such as nicotine show mild short-term improvements, while sedatives and antihistamines significantly impair RT.
Practice & Training
Consistent practice can improve reaction time by 10–20% over weeks. Training strengthens specific neural pathways through a process called myelination, which increases signal transmission speed. Athletes, gamers, and musicians all show faster reaction times in their domains of expertise. The improvement is partly task-specific, but general processing speed can also benefit.
Physical Fitness
Regular aerobic exercise is associated with faster reaction times, particularly in older adults. Exercise increases cerebral blood flow, promotes neuroplasticity, and elevates levels of brain-derived neurotrophic factor (BDNF). Studies show that physically active individuals have reaction times 10–15% faster than sedentary controls, even after controlling for age and other factors.
Reaction Time by Stimulus Type (Simple RT, Young Adults)
| Stimulus | Average RT | Range | Primary Pathway |
|---|---|---|---|
| Auditory (sound) | ~140 ms | 120–170 ms | Cochlea → Brainstem → Auditory cortex |
| Tactile (touch) | ~155 ms | 130–180 ms | Mechanoreceptors → Spinal cord → Somatosensory cortex |
| Visual (light) | ~180 ms | 150–220 ms | Retina → Thalamus (LGN) → Visual cortex |
Historical Measurement
Helmholtz and Nerve Conduction Speed
Hermann von Helmholtz was the first to measure the speed of nerve impulses, using frog legs and later human subjects. By stimulating nerves at different distances from the muscle and measuring the delay, he calculated nerve conduction velocity at roughly 27 meters per second — proving that neural signals were not instantaneous, as previously assumed. This was one of the foundational experiments in neuroscience.
Donders' Subtraction Method
Franciscus Donders proposed a clever technique: by comparing simple, recognition, and choice reaction times, one could isolate the duration of specific cognitive stages. Subtracting simple RT from choice RT estimates the time for stimulus identification and response selection. Though the pure subtraction assumption has limitations, Donders' method remains influential and laid the groundwork for modern cognitive chronometry.
Modern Computerized Testing
Today, reaction time is measured with millisecond precision using computerized testing platforms, EEG, and fMRI. Digital tools like reaction-time-test.io make it possible for anyone to measure their own RT. Research applications range from clinical neuropsychological assessment to sports performance analysis and human-computer interaction design.
Reaction Time in Real Life
Reaction time is not just a lab measurement — it determines outcomes in safety-critical situations, athletic competition, and high-performance domains every day.
Driving
The total brake reaction time for drivers averages about 1.5 seconds, which includes perception time (~0.5s), cognitive decision time (~0.3s), and movement time to reach the brake pedal (~0.7s). At 60 mph (97 km/h), a car travels 132 feet (40 meters) during that 1.5 seconds. This is why following distance and alertness are so critical to road safety.
Sports
In baseball, a batter has roughly 400 ms to see a 95-mph fastball, decide whether to swing, and execute the motion. A tennis player returning a 130-mph serve has even less time. Elite goalkeepers in soccer begin their dive based on the kicker's body cues before the ball is struck, using anticipatory reaction rather than pure reflexive response. You can practice similar skills with our F1 reaction test.
Gaming & Esports
Professional esports players typically achieve visual reaction times of 150–180 ms, placing them well above average. In fast-paced games like Counter-Strike or Valorant, the difference between a 180 ms and 220 ms reaction time can determine who wins a gunfight. Pro gamers train their reactions daily, much like traditional athletes. Try our aim reaction test to see how you compare.
Aviation & Military
Fighter pilots must react to rapidly changing situations while processing multiple instrument readings, radio communications, and visual targets. Military reaction time testing is part of pilot selection criteria. Aviation studies show that even small improvements in RT — 20 to 30 ms — can be operationally significant when decisions must be made at speeds exceeding 1,500 mph.
Test These Concepts Yourself
Now that you understand the science, put your nervous system to the test. Measure your own reaction time, compare your results to population averages, and discover strategies to improve your reaction time.
Frequently Asked Questions
What is the fastest possible human reaction time?
The fastest reliably recorded human reaction times to visual stimuli are around 120 milliseconds, though most athletic governing bodies treat anything below 100 ms as a false start. Auditory reaction times can be slightly faster, with elite sprinters reaching approximately 110–120 ms. The absolute physiological floor is estimated at around 80–90 ms, limited by the minimum time required for sensory transduction, neural transmission, and motor activation.
Why is auditory reaction time faster than visual?
Auditory reaction time (~140 ms) is faster than visual reaction time (~180 ms) because the auditory pathway involves fewer synaptic connections between the ear and the motor cortex. Sound signals are converted to electrical impulses by the cochlea more rapidly than light is processed by the retina, and the brainstem can initiate motor responses to sound before full cortical processing is complete. This ~40 ms difference is why sprinters respond to a starting gun rather than a starting flash.
What part of the brain controls reaction time?
Reaction time involves multiple brain regions working in concert. The primary sensory cortices (visual cortex in the occipital lobe, auditory cortex in the temporal lobe) process the incoming stimulus. The prefrontal cortex handles decision-making and response selection. The premotor and primary motor cortices plan and execute the movement. The cerebellum coordinates precise timing, and the basal ganglia help initiate voluntary movement. The speed of communication between these areas — mediated by white matter tracts — is a major determinant of your overall reaction time.
Is reaction time genetic?
Research suggests that genetics account for approximately 25–50% of the variation in reaction time between individuals. Heritable factors include nerve conduction velocity, neurotransmitter receptor density, and baseline neural processing efficiency. Twin studies show higher RT correlation between identical twins than fraternal twins. However, the remaining variance is attributable to training, lifestyle, sleep, fitness, and other modifiable factors — meaning there is substantial room for improvement regardless of your genetic baseline.
How does reaction time change during the day?
Reaction time follows a circadian rhythm and is typically slowest shortly after waking and in the late evening. Most people reach peak reaction speed in the late morning to early afternoon (roughly 10 am to 2 pm), when core body temperature and cortical alertness are highest. Reaction time can also degrade after meals due to postprandial blood flow redistribution, and it worsens dramatically with accumulated sleep debt. For the most accurate personal benchmark, test yourself at the same time each day.
Can reaction time predict cognitive decline?
Yes, research shows that reaction time is a sensitive marker of cognitive health. Longitudinal studies have found that slowing reaction times can precede clinical diagnosis of conditions like Alzheimer's disease and other dementias by several years. Increased intra-individual variability — inconsistency across trials — is an even stronger predictor than average speed alone. However, reaction time is one indicator among many used in cognitive assessment, and a single slow score should not be a cause for concern. Regular monitoring over time is more informative.
Does reaction time differ between men and women?
On average, studies show a small but consistent difference: males tend to have reaction times approximately 10–20 ms faster than females for both auditory and visual stimuli. This difference is thought to relate to factors like average nerve conduction velocity and hormonal influences on neural processing. However, the distributions overlap substantially, and trained females frequently outperform untrained males. See average reaction time data for more details.
What is the difference between reaction time and response time?
Reaction time (RT) measures the interval from stimulus onset to the initiation of a response — the moment you begin to move. Response time includes reaction time plus movement time — the duration of the physical action itself (e.g., moving your foot from the gas pedal to the brake). In driving safety research, the total response time (~1.5 seconds) is the critical metric, but in laboratory settings and online testing, the purer reaction time measure is typically used.