Does Drinking Water Make You Smarter? What Dehydration Does to IQ

Drinking water does not make you smarter: no study has shown that extra water raises IQ, and no study has given a full IQ test to people in different states of hydration. What the research does show is that dehydration can slightly worsen performance on some cognitive tasks, with a 2018 meta-analysis finding a small overall impairment (about a fifth of a standard deviation) that was larger for attention tasks and for fluid losses above 2% of body weight.

This page covers the adult and child studies, why they disagree, and where the "eight glasses a day" rule came from. It describes research and is not medical advice.

Researchers usually express dehydration as the percentage of body weight lost as water. A person who weighs 70 kg and sweats away 1.4 kg without drinking has lost 2% of body mass. In the studies below, losses of that size were usually produced by exercise, heat or fluid restriction.

The largest summary is a meta-analysis by Wittbrodt and Millard-Stafford in Medicine and Science in Sports and Exercise. They pooled 33 studies with 413 participants, whose fluid losses ranged from 1% to 6% of body mass, and calculated 280 separate effect sizes.

• Overall effect: Across all tasks, dehydration impaired performance by a standardized mean difference of 0.21. That is a small effect.

• Attention: Attention tasks showed the largest impairment, 0.52 of a standard deviation.

• Executive function and motor coordination: Executive function tasks (0.24) and motor coordination tasks (0.40) were also worse.

• Reaction time: Reaction-time tasks barely changed (0.10, not statistically significant).

• Size of the fluid loss: Studies with losses above 2% of body mass showed more impairment (0.28) than studies at 2% or below (0.14, with a confidence interval that reached zero).

A standardized mean difference expresses a gap between conditions in standard deviation units. Multiplying by 15 gives a sense of scale on the familiar IQ metric: 0.21 would be roughly 3 points and 0.52 roughly 8 points. Those numbers describe scores on narrow laboratory tasks. They are not IQ points.

A second meta-analysis reached a weaker conclusion. Goodman, Moreland and Marino restricted their analysis to "active" dehydration, produced by exercise or heat, and found no significant overall impairment (g = -0.18, with a 95% confidence interval from -0.53 to 0.18). Complex attention, executive function, learning and memory were not significantly affected, whether or not fluid loss passed 2%. Two careful reviews reaching different verdicts is itself informative: whatever the effect is, it is small and depends on how a study is run.

Two linked studies tried to separate dehydration from overheating. Ganio and colleagues had 26 young men walk on a treadmill in a warm room while dehydrated (about 1.6% of body mass, sometimes with a diuretic drug) or while drinking enough to stay hydrated. When dehydrated, the men made more errors on a visual vigilance task and responded more slowly on a visual working memory task. They also reported more fatigue and tension.

Armstrong and colleagues ran the same design with 25 young women at about 1.4% dehydration. The women reported lower concentration, more headaches, a worse mood and found the tasks harder, but most measures of cognitive performance did not change. Feeling worse did not always mean performing worse.

Children are the group in which the most consistent findings have appeared. In a 2011 review, David Benton noted that four intervention studies had found better performance in children aged 7 to 9 after a drink of water, even though the children were eating and drinking normally beforehand.

• Edmonds and Burford (2009): Fifty-eight children aged 7 to 9 were randomly assigned to receive extra water or no extra water. The water group rated themselves less thirsty and did better on a letter-cancellation task and on "spot the difference" memory tasks.

• Edmonds and Jeffes (2009): Twenty-three children aged 6 and 7 improved more from baseline on visual attention and visual search after a drink of water, but not on visual memory or a visuomotor task.

• Benton and Burgess (2009): Forty children (average age 8 years 7 months) were tested in the afternoon on two days, once after drinking 300 ml of water. Recall of objects was better after water, while sustained attention did not differ.

• Khan and colleagues (2019): In a larger crossover trial, 75 children aged 9 to 11 drank either their usual amount, a low amount (0.5 L a day) or a high amount (2.5 L a day) for four days each. The high-water condition improved performance on a task-switching measure, with 34% lower working memory cost than the low-water condition. Two other cognitive tasks showed no change.

These studies are small, and each one measured several tasks, so a few positive results could appear by chance. None measured IQ; the outcomes were brief attention and memory tasks. The most reasonable reading is that a slightly thirsty child may concentrate a little better after a drink, a short-term effect that says nothing about long-term intellectual ability.

Hydration research faces some unusual problems, and they help explain why studies disagree.

• Heat and exercise: Most adult studies make people dehydrated by having them exercise, often in heat. Fatigue and a raised body temperature can impair thinking on their own, so it is hard to isolate the effect of water loss. Benton concluded that in young adults there is little evidence that dehydration under normal living conditions disrupts cognition.

• Participants know: It is impossible to hide from someone whether they have had a drink. Expectations about feeling sluggish when thirsty can shape both mood ratings and effort on a test.

• Thirst itself: Edmonds, Crombie and Gardner tested 34 adults after an overnight fast without fluids. A 500 ml drink of water sped up simple reaction time, but only for people who felt quite thirsty. For people who did not feel thirsty, water made no difference. The authors suggested that the sensation of thirst may take up some mental resources, which a drink then frees.

• Older adults: In a Spanish study of 1,957 adults aged 55 to 75 with overweight and metabolic syndrome, Nishi and colleagues found that higher serum osmolarity (a blood marker of lower hydration) at the start predicted a slightly faster decline in a global cognitive score over two years. Reported water intake did not predict decline. As an observational result, it cannot show that low hydration caused the change, because illness and medication can affect both.

Short-term state is a familiar theme in testing. Our page on factors that affect your IQ test results covers sleep, stress, illness and other conditions that can pull a score below a person's typical level without changing the underlying ability.

The familiar advice to drink eight 8-ounce glasses a day has a weak scientific basis. The physiologist Heinz Valtin searched for the origin of "8 x 8" in 2002 and found no scientific studies supporting it for healthy adults in temperate climates. He noted that caffeinated drinks and food also supply water.

The US Institute of Medicine (now the National Academy of Medicine) reached a similar view in its 2004 report on water. It set an "adequate intake" of about 2.7 liters of total water a day for women and 3.7 liters for men, counting water from all drinks and food, About 80% comes from drinks, including coffee and tea, and the panel said that most healthy people meet their needs by letting thirst be their guide. It declined to give a number of glasses. People who are very active or live in hot climates need more. If you drink coffee for alertness, our page on caffeine and IQ covers what that does and does not change.

More water is not always better. Drinking far beyond need can dilute the sodium in the blood, a condition called "hyponatremia." In a study of the 2002 Boston Marathon, Almond and colleagues found that 13% of 488 runners who gave a blood sample at the finish had hyponatremia, and 0.6% had a critical level. Weight gain during the race, a sign of drinking more than was lost in sweat, was a major risk factor. Hyponatremia can cause confusion, seizures and, in rare cases, death. People with kidney or heart conditions, or taking medicines that affect fluid balance, should ask a doctor or pharmacist how much to drink, and sudden confusion in an older person is a reason to seek medical care promptly.

No. No study shows that drinking water raises IQ. Being thirsty or more than mildly dehydrated can slightly impair attention and mood, and a drink can restore that short-term performance.

No study has measured IQ in dehydrated people. A 2018 meta-analysis found that dehydration lowered scores on cognitive tasks by about 0.21 standard deviations overall, with larger effects on attention and when fluid loss exceeded 2% of body weight.

There is no evidence that extra water improves an IQ score. Arriving comfortable, rested and not thirsty is sensible for any test, because thirst can be a distraction.

Several small studies of 6- to 11-year-olds found better attention or memory scores shortly after a drink of water. The effects were short-term and on brief tasks, and they have not been linked to IQ or long-term achievement.

There is no scientific basis for a fixed eight glasses. US guidance sets total water intake from drinks and food at about 2.7 liters a day for women and 3.7 liters for men, and says thirst is a good guide for most healthy people.

Water does not make anyone smarter. Dehydration, especially beyond about 2% of body weight, can modestly worsen attention and mood, and thirsty children may concentrate a little better after a drink, but none of this research involves IQ, and the effects fade once fluid is replaced. Drinking when thirsty matters far more than chasing a daily glass count. If you want an accurate picture of your reasoning ability, take the RIOT IQ test.

1. Wittbrodt, M. T., & Millard-Stafford, M. (2018). Dehydration impairs cognitive performance: A meta-analysis. Medicine and Science in Sports and Exercise, 50(11), 2360-2368. [doi.org/10.1249/MSS.0000000000001682](. doi.org

2. Goodman, S. P. J., Moreland, A. T., & Marino, F. E. (2019). The effect of active hypohydration on cognitive function: A systematic review and meta-analysis. Physiology & Behavior, 204, 297-308. [doi.org/10.1016/j.physbeh.2019.03.008](. doi.org

3. Ganio, M. S., Armstrong, L. E., Casa, D. J., McDermott, B. P., Lee, E. C., Yamamoto, L. M., Marzano, S., Lopez, R. M., Jimenez, L., Le Bellego, L., Chevillotte, E., & Lieberman, H. R. (2011). Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition, 106(10), 1535-1543. [doi.org/10.1017/S0007114511002005](. doi.org

4. Armstrong, L. E., Ganio, M. S., Casa, D. J., Lee, E. C., McDermott, B. P., Klau, J. F., Jimenez, L., Le Bellego, L., Chevillotte, E., & Lieberman, H. R. (2012). Mild dehydration affects mood in healthy young women. Journal of Nutrition, 142(2), 382-388. [doi.org/10.3945/jn.111.142000](. doi.org

5. Benton, D. (2011). Dehydration influences mood and cognition: A plausible hypothesis? Nutrients, 3(5), 555-573. [doi.org/10.3390/nu3050555](. doi.org

6. Edmonds, C. J., & Burford, D. (2009). Should children drink more water? The effects of drinking water on cognition in children. Appetite, 52(3), 776-779. [doi.org/10.1016/j.appet.2009.02.010](. doi.org

7. Edmonds, C. J., & Jeffes, B. (2009). Does having a drink help you think? 6-7-year-old children show improvements in cognitive performance from baseline to test after having a drink of water. Appetite, 53(3), 469-472. [doi.org/10.1016/j.appet.2009.10.002](. doi.org

8. Benton, D., & Burgess, N. (2009). The effect of the consumption of water on the memory and attention of children. Appetite, 53(1), 143-146. [doi.org/10.1016/j.appet.2009.05.006](. doi.org

9. Khan, N. A., Westfall, D. R., Jones, A. R., Sinn, M. A., Bottin, J. H., Perrier, E. T., & Hillman, C. H. (2019). A 4-d water intake intervention increases hydration and cognitive flexibility among preadolescent children. Journal of Nutrition, 149(12), 2255-2264. [doi.org/10.1093/jn/nxz206](. doi.org

10. Edmonds, C. J., Crombie, R., & Gardner, M. R. (2013). Subjective thirst moderates changes in speed of responding associated with water consumption. Frontiers in Human Neuroscience, 7, 363. [doi.org/10.3389/fnhum.2013.00363](. doi.org

11. Nishi, S. K., Babio, N., Paz-Graniel, I., Serra-Majem, L., Vioque, J., Fitó, M., Corella, D., Pintó, X., Bueno-Cavanillas, A., Tur, J. A., et al. (2023). Water intake, hydration status and 2-year changes in cognitive performance: A prospective cohort study. BMC Medicine, 21(1), 82. [doi.org/10.1186/s12916-023-02771-4](. doi.org

12. Valtin, H. (2002). "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 283(5), R993-R1004. [doi.org/10.1152/ajpregu.00365.2002](. doi.org

13. Institute of Medicine. (2005). Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. National Academies Press. [doi.org/10.17226/10925](. doi.org

14. National Academies. (2004, February 11). Report sets dietary intake levels for water, salt, and potassium to maintain health and reduce chronic disease risk [News release]. [nationalacademies.org](. nationalacademies.org

15. Almond, C. S. D., Shin, A. Y., Fortescue, E. B., Mannix, R. C., Wypij, D., Binstadt, B. A., Duncan, C. N., Olson, D. P., Salerno, A. E., Newburger, J. W., & Greenes, D. S. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 352(15), 1550-1556. [doi.org/10.1056/NEJMoa043901](. doi.org

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