Reaction time gets treated like a fixed personality trait, but it's anything but. Your score on a reaction time test can swing by 50 milliseconds between two attempts on the same day, and the cause is almost never your nervous system. It's usually something mundane — how much sleep you got last night, what mouse you're using, whether you've had a coffee.
This post breaks down the factors that genuinely move the needle, ranked roughly by how much they matter. Some you can control. Some you can't. Knowing which is which saves you from chasing gains that aren't there.
The big three: sleep, age, and hardware
If you only pay attention to three things, make it these. They account for the vast majority of variation between individuals and between sessions.
- Sleep — one night of 5 hours adds 10–20 ms. Chronic sleep debt of 2+ hours per night adds 30+ ms. This is the single biggest controllable factor.
- Age — reaction time peaks at 17–19 and declines by about 1–2 ms per year after 25. By 65, most adults have lost 60–80 ms relative to their teenage peak.
- Hardware — a trackpad adds 30–50 ms of latency over a wired mouse. A 60Hz monitor adds 7–16 ms over a 144Hz panel. Bluetooth mice sit somewhere in between.
Notice that two of those three are fully controllable. You can't un-age yourself, but you can sleep more and you can buy a $25 wired mouse. If your reaction time score is bad, fix these first.
The medium four: caffeine, hydration, fatigue, screen brightness
These each move your score by 5–15 ms. Individually they're small. Stacked together they can add up to 40 ms — which is the difference between an average score and a fast one.
Caffeine is the most studied. A moderate dose (100–200 mg, roughly one to two coffees) drops RT by 5–10 ms, with the effect peaking 30–60 minutes after consumption. Higher doses don't help further and can cause jitteriness that actually slows you down. Tolerance builds in daily users, so if you drink coffee every morning, the caffeine isn't doing much for your RT anymore.
Hydration matters more than people expect. Even mild dehydration — losing 2% of body weight as water, which most office workers hit by mid-afternoon — measurably slows RT. Drinking a glass of water before a test reliably helps if you're even slightly dehydrated.
Fatigue is the cumulative version of sleep debt. If you've been staring at a screen for 8 hours, your RT will be 10–20 ms slower than in the morning, regardless of how well you slept. Taking a 10-minute break with eyes closed recovers most of this.
Screen brightness affects pupil response time. Dim screens slow your optic nerve's response to the colour change. Crank the brightness up — not to the point of discomfort, but enough that the panel is clearly brighter than the surrounding room.
The small stuff: temperature, time of day, mood
These each move your score by 2–8 ms. Real, but barely noticeable outside of lab settings.
Temperature: cold rooms slow nerve conduction. Below about 18°C (65°F), your RT will start to creep up. Warm rooms help slightly, up to about 24°C (75°F), after which heat-induced drowsiness takes over.
Time of day: most people are fastest between 10am and 1pm, and slowest between 3am and 6am. The swing is about 8 ms. If you're tracking changes over time, test at the same hour each session.
Mood: anxiety and stress slightly speed up simple RT (heightened arousal) but slow down choice RT (impaired decision-making). The effects are small and noisy.
What doesn't matter (much)
A lot of conventional wisdom about reaction time is wrong, or at least overstated. Here's what people worry about that they probably shouldn't.
- Gender — men are about 15–20 ms faster on average, but individual variation dwarfs the group difference. Knowing someone's gender tells you almost nothing about their personal RT.
- Diet — beyond staying hydrated and avoiding heavy meals before testing, diet has minimal short-term effect on RT. Long-term cardiovascular health matters; specific 'brain foods' don't.
- Intelligence — there's a weak correlation (around -0.3) between IQ and choice RT, but it's not strong enough to be useful for any individual.
- Handedness — left-handed vs right-handed makes no measurable difference on simple RT. Choice RT shows tiny effects on handedness-specific tasks, but it's negligible.
- Eye color — yes, there are studies claiming lighter-eyed people have faster RTs. The effect, if real, is well under 5 ms. Ignore.
How to use this list
If you want to drop your reaction time score, work through the list from top to bottom. Sleep first. Hardware second. Caffeine and hydration third. Skip the small stuff until the big stuff is sorted.
If you're tracking changes over time, control the controllables. Test at the same time of day, in the same room, on the same hardware, with similar caffeine intake. Otherwise you're measuring noise.
And remember — reaction time is just one of many cognitive and physical traits. Being 30 ms faster on a test will not change your life. The things that improve RT (sleep, exercise, hydration) improve almost everything else too, which is the more honest reason to focus on them.