Hydration And Reaction Time
Lose 2% of body water and you measurably slow down.
TL;DR — A 2% drop in body water (the level you reach before feeling thirsty) slows reaction time by 10–25 ms. Drinking 500 ml of water restores baseline within 30–60 minutes.
Thirst is a lagging indicator. By the time you feel it, your RT is already 10+ ms slower.
The threshold
Ganio et al. (2011) and Adan (2012) independently found measurable cognitive impairment beginning at 1.5–2% body-mass loss from dehydration. Reaction time slows by 8–20 ms; mood and attention degrade similarly.
What 2% looks like in practice
- 70 kg adult: 1.4 kg water deficit ≈ skipping water for ~5 hours in summer.
- A long gaming session in a warm room can hit 1.5% in 3–4 hours.
- Coffee and alcohol both accelerate the drop.
Rehydration timeline
| Action | Time to RT recovery |
|---|---|
| 500 ml water | 30–60 min |
| Electrolyte drink (post-cardio) | 20–40 min |
| Slow sipping 1 L over hour | ~45 min |
| Coffee only | partial — worsens net deficit |
Optimal session strategy
- Drink 400–500 ml in the 60 minutes before play.
- Sip 150–200 ml every 30 minutes during long sessions.
- Add a pinch of salt or an electrolyte tab if sweating or in heat.
Frequently Asked Questions
Does overdrinking help?
No. Beyond euhydration the RT benefit plateaus and bathroom breaks become disruptive.
Do sports drinks beat water?
Only when you have been sweating or playing 90+ minutes.
Does coffee count toward hydration?
Mostly yes at moderate doses, but the diuretic effect partly offsets it.
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