We’ve all heard the advice to “drink more water.” But what does the research actually say about how changes in water intake affect our mood, and does it matter how much you habitually drink in the first place? A landmark peer-reviewed study published in PLOS ONE (Pross et al., 2014) addressed exactly this question, yielding striking findings that challenge a one-size-fits-all approach to hydration.
Who Are “High” and “Low” Drinkers?
Before diving into the findings, it helps to understand how researchers defined their groups. The study recruited habitual high-volume drinkers (HIGH), defined as those with fluid intakes of at least 2 liters per day, and low-volume drinkers (LOW), defined as those consuming less than 1.2 liters per day. These weren’t extreme outliers; they represent a real spectrum of everyday drinking habits found in the general population.
During a two-day baseline period, HIGH participants consumed 2.5 liters of water per day, and LOW participants consumed 1 liter per day. Over the following three controlled intervention days, HIGH had their water intake restricted to 1 liter per day, while LOW had theirs increased to 2.5 liters per day. To measure mood, researchers administered several validated scales, including the Bond & Lader Visual Analog Scale, the Profile of Mood States (POMS), the Karolinska Sleepiness Scale, and a Thirst & Emotional Visual Analog Scale, at multiple time points throughout each day.
What Happens When High Drinkers Cut Back?
The results for the HIGH group were sobering. Restricting water intake in high drinkers led to a significant increase in thirst and decreases in contentedness, calmness, positive emotions, and vigor/activity. In other words, it wasn’t just physical discomfort; their emotional landscape shifted meaningfully in a negative direction.
Habitual high drinkers who were forced to reduce their daily water intake reported being thirstier, less calm, less content, less vigorous and described experiencing fewer positive emotions. These weren’t trivial fluctuations. The changes in calmness and positive emotion reached statistical significance, suggesting a genuine psychological cost to under-hydrating when your body is accustomed to more.
The researchers had anticipated this outcome based on prior dehydration literature. It was hypothesized that a water decrease in HIGH would negatively impact sleep/wake-related moods, consistent with the effects seen in fluid deprivation studies. The data bore this out and extended the finding beyond just sleepiness into broader emotional well-being.
What Happens When Low Drinkers Drink More?
The LOW group told a different, but equally compelling, story. The intervention demonstrated that a daily water intake increase led to a significant mood improvement in habitual low drinkers, who reported less fatigue, less confusion, less thirst, and who tended to be less sleepy.
Specifically, increased water consumption in the LOW group resulted in a significant decrease in confusion bewilderment scores, and lower sleepiness was also observed as a trend. For people who chronically underdrink, simply bringing their intake up to a more adequate level appears to have a meaningful positive effect on how alert, clear-headed, and energized they feel day-to-day.
The hypothesis had been that increased water intake would positively affect alertness and happiness ratings in LOW, as previously demonstrated in adults and young children, and the results supported this expectation.
The Bigger Picture: Why Does Water Affect Mood at All?
This study doesn’t stand alone. A broader body of peer-reviewed evidence connects hydration status to mental and emotional function.
When dehydration reduces body mass by more than 2%, it has been consistently reported that mood is influenced, fatigue is greater, and alertness is lower, though the effects on cognition have been less consistent.
Current findings in the field suggest that particular cognitive abilities and mood states are positively influenced by water consumption, with the impact of dehydration on cognition and mood being particularly relevant for those with poor fluid regulation, such as the elderly and children.
Dehydration of 2% or more body mass loss can increase perception of fatigue, induce impairment in cognitive functions such as memory and attention, and alter mood. Researchers have also begun investigating the consequences of mild dehydration, well below the 2% threshold, that many people may be experiencing in their daily lives without realizing it.
On the neurological side, despite the lack of behavioral changes in cognition, neural activity in brain regions involved in attention and executive function has been shown to increase when individuals are mildly dehydrated compared to when they are fully hydrated, suggesting the brain works harder just to maintain normal function when water is insufficient.
Key Takeaways: It’s Not Just “Drink More Water”
One of the most important insights from the Pross et al. study is that hydration recommendations should be personalized. The results suggest that an increase or decrease in habitual water intake has, respectively, an improving or worsening effect on mood and sensations, depending upon an individual’s habitual drinking habits.
Put simply: if you normally drink a lot of water, cutting back, even moderately, can dampen your mood in ways you might not immediately associate with thirst. And if you’re a habitually light drinker, simply drinking more can lift the mental fog and fatigue you may have learned to accept as normal.
Increasing water intake has a beneficial mood effect, especially on sleep/wake feelings, whereas restricting water intake can deteriorate mood, and even though the results were observed in young adults, they can still be of importance to the healthy aging population.
Conclusion
The science of hydration and mood is still evolving, but the evidence is clear enough to take seriously: water intake shapes how we feel, and the effect depends on what your body is used to. Whether you’re a habitual high drinker whose mood dips when you can’t get enough, or a low drinker who has quietly lived with unnecessary mental fatigue, your water glass may be more powerful than you think. The next time you feel irritable, foggy, or worn out, it might be worth asking when you last had a drink of water.
References
Pross, N., Demazières, A., Girard, N., Barnouin, R., Metzger, D., Klein, A., Perrier, E., & Guelinckx, I. (2014). Effects of changes in water intake on mood of high and low drinkers. PLoS ONE, 9(4). https://doi.org/10.1371/journal.pone.0094754
Masento, N. A., Golightly, M., Field, D. T., Butler, L. T., & van Reekum, C. M. (2014). Effects of hydration status on cognitive performance and Mood. British Journal of Nutrition, 111(10), 1841–1852. https://doi.org/10.1017/s0007114513004455
Benton, D., & Young, H. A. (2015). Do small differences in hydration status affect mood and mental performance? Nutrition Reviews, 73(suppl 2), 83–96. https://doi.org/10.1093/nutrit/nuv045
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.
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