Reaction Time: What It Is and Why It Changes With Age

Reaction time is the elapsed time between a stimulus and a voluntary response. Simple reaction time — pressing a button when a light appears — measures the minimum signal transmission time through the perceptual and motor systems. Choice reaction time — pressing different buttons depending on which of two stimuli appears — adds decision-making to the measurement and is substantially more sensitive to cognitive aging than simple reaction time.

The neural components of reaction time span the full signal pathway: sensory detection in the relevant cortex, signal transmission through white matter pathways, decision processing in the prefrontal and parietal cortices, motor planning in the supplementary motor area and cerebellum, and signal transmission down the corticospinal tract to the executing muscles. Slowing anywhere in this chain delays the response.

Reaction time has been studied as a cognitive measure for over a century, going back to Francis Galton's 1880s anthropometric laboratory. It correlates strongly with general intelligence, health outcomes, and longevity. A 2015 meta-analysis by Stuart Ritchie and colleagues found that slower reaction time in midlife predicts faster cognitive decline and earlier mortality — making it one of the most pragmatically useful measures in aging research.

Reaction time slows with age reliably and consistently. Simple reaction times of 250 milliseconds in young adults typically extend to 400 milliseconds or more by age 70. Choice reaction times slow at roughly twice the rate. The slowing reflects the cumulative effect of reduced myelination, lower dopamine signaling in the basal ganglia and prefrontal cortex, reduced cerebellar precision, and decreased signal conduction velocity throughout the nervous system.

Importantly, reaction time slowing is not entirely about physical speed. Much of the age-related extension comes from the decision-making component — older adults show greater variability in response timing and tend toward more cautious decision thresholds, trading speed for accuracy in ways that are adaptive but measurable. This decision-level slowing is cognitively driven, not purely motor.

Elite athletes show slower reaction times than they did in their 20s even when physically fit. Professional tennis players, combat athletes, and racing drivers all show measurable reaction time increases by their mid-30s. This is one of the reasons sports that rely most heavily on reaction speed impose natural career windows that no amount of training fully offsets.

Acute reaction time slowing is a well-validated marker of drowsiness and intoxication. The psychomotor vigilance task — a sustained reaction time test — is the gold standard measure of sleep deprivation's cognitive effects. A single night of four hours of sleep slows reaction time and increases response variability to levels comparable to legal alcohol impairment. This has obvious safety implications for driving and operating equipment.

Persistent slowing beyond what would be expected for age, particularly when accompanied by increased variability (inconsistent response times rather than uniformly slow ones), is associated with conditions including Parkinson's disease, vascular cognitive impairment, and early Alzheimer's disease. The characteristic of increased variability — not just slower responses but more inconsistent responses — is a particularly sensitive marker of early neurodegeneration across multiple conditions.

Keel measures choice reaction time as a component of its processing speed task, capturing both the central tendency (your typical response speed) and the variability of your responses across trials. Variability is as informative as average speed — someone who is consistently slow shows a different pattern than someone who alternates between fast and very slow responses.

Your reaction time trend is sensitive to sleep, stress, and illness, which makes it excellent for detecting day-to-day variation and helping you understand what affects your brain. It is also sensitive to the subtle neurological changes that precede overt cognitive decline, which is why maintaining a long-term personal baseline is valuable — you would know your own normal and could detect a meaningful departure from it.