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Water is arguably the best drink on Earth. Drinking water provides undisputable benefits to humans, other animals and plants. We know it’s possible to overwater a plant, but what about us humans? Can we drink too much water? And is it worse to be over-hydrated or under-hydrated?

“Many, many more people die from over-hydration than under-hydration,” said Mark Knepper, MD, PhD, of NIH’s National Heart, Lung and Blood Institute in a 2009 APS podcast. Hyponatremia—or “water intoxication”—is a condition that develops when the sodium in our bodies becomes diluted. According to the Mayo Clinic, with hyponatremia “your body’s water levels rise and your cells begin to swell. This swelling can cause many health problems, from mild to life-threatening.” Dr. Knepper and other experts have found that drinking water when thirsty is the best way to maintain optimal hydration.

Even during times of prolonged exercise or extended workouts in hot weather, the recommendation is the same. “Just drink water when you’re thirsty like a normal person,” says a Huffington Post article on guidelines released earlier this month by the International Exercise-Associated Hyponatremia (EAH) Consensus Development Conference. The panel of experts found the best way to find a happy hydration level is to use thirst as your guide.

If you’re worried about dehydration from skipping a few sips, don’t. Dr. Knepper suggested that experiencing periods of moderate dehydration is good for the kidneys. “The bottom line is you need to condition your kidneys just like you condition your muscles.” This helps keep the kidneys, and the hormones that regulate them, working at optimal levels. Chronic over-hydration can suppress stores of vasopressin—a hormone that regulates the amount of water that your kidneys excrete.

Still wondering about the best water level for you? Take this advice from Heinz Valtin, MD, a leading authority in water intake. His 2002 review article on the “8 x 8” rule (eight 8-oz. glasses of water per day) changed the conventional wisdom on how much water we need to maintain optimal health. “The best rule I can give is: Drink what you do customarily and between meals, plus when you’re thirsty. You’ll be all right.”

Stacy Brooks is the former director of marketing and communications for the American Physiological Society (APS). One of her favorite things about working at APS was learning about the interesting and important research that physiologists do and finding ways to communicate their science to a wide variety of audiences who benefit from these research advances.