Brain Drain

Your brain, which is about 75 percent water, is especially sensitive to dips in hydration. Without enough fluid circulating throughout, your body doesn’t function as efficiently—potentially impacting clarity, reaction time, and mood, explains Caroline Passerrello, EdD, RDN, a spokesperson for the Academy of Nutrition and Dietetics.

“When your body is well hydrated, the brain communicates more efficiently,” adds Andie Lee Gonzalez, RDN, a scientist at Abbott, a global healthcare company.

A review article in the British Journal of Nutrition found that mild, short-term dehydration most clearly impacts a few specific cognitive abilities: sustained visual attention (find-the-differences puzzles, driving, and reading), short-term memory, and reaction time. In the studies reviewed, scores on tests of these skills improved after participants drank water. Another study in the same journal found healthy adults who restricted their water intake reported headaches, decreased ability to concentrate, and reduced alertness.

Simply put, dehydration is when your body doesn’t have enough water to function properly. Here’s how to stay hydrated—and avoid that afternoon slump.

How Much to Drink

Fluid needs are individualized. Even expert recommendations vary. “Think of hydration the way you think of sleep: essential but not one-size-fits-all,” Gonzalez says. “Most adults feel their best drinking somewhere between 2 and 3 liters of fluid daily.” That’s about 8 to 12 cups.

Passerrello suggests 100 ounces per day for women and 125 ounces for men—“simple, round numbers that align with dietary guidance,” she says. (That’s around 12.5 cups for women and 15.5 cups for men.)

If those numbers sound ambitious, don’t fixate on perfection. Gonzalez says it’s less about hitting a specific number and more about staying consistently hydrated.

It All Adds Up

Water isn’t your only source for hydration: “All fluids count toward your daily hydration, even coffee and tea,” Gonzalez says. Milk, juice, sparkling water, kombucha, soda, and sports drinks all contribute (though sugary drinks, with their added calories, should not be the foundation of your fluid intake).

Food is a source of hydration too. About 20 percent of daily fluids typically come from what you eat. In particular, fruits, vegetables, and grains—which cook in water, absorbing it for you to eat later—contribute to our hydration status even if we don’t drink a lot of water, Planells says.

When to Drink More

The amount of fluid your body requires fluctuates with age, activity levels, climate, medications, and illness—particularly fever, vomiting, or diarrhea—to name a handful.

“If you’re sweating for any reason, you’re losing fluids,” Passerrello says. Heat, humidity, exercise, and even high altitudes increase loss. We also lose water simply by breathing—more so in cold or dry air, she adds.

Air travel is an often overlooked drain. “Airplane cabin air can be incredibly dry, with humidity levels often hovering around 10 to 20 percent, which is drier than most deserts,” Gonzalez says. “That means you may lose more moisture simply by breathing, so drinking extra water on flights may help you feel noticeably better when you land.”

Am I Hydrated?

Under typical conditions and moderate activity, your body maintains its water balance remarkably well: The feeling of thirst is a late indication your body needs more water, while your kidneys, skin, lungs, and—at a very low level—digestive system remove water if you have excess. A quick check-in can give you a good idea of how well you’re hydrating.

“If you feel energized, your lips and skin don’t feel dry, and your urine is a pale, lemonadelike color, you’re likely well hydrated,” Gonzalez says.

Signs of underhydration include fatigue, headaches, dry mouth, darker urine, and thirst.

Drink Up

Water consumption is an easily overlooked but powerful element in your wellness routine. As Passerrello puts it, “Hydration isn’t a wellness trend; it’s foundational to health.”