We know that lack of food can make a person say and take bad decisions. But what about lack of water?

According to a new study by Georgia Institute of Technology looking into the effects of dehydration, too little water can dampen cognitive skills, too. And it’s not just in extreme cases of dehydration, after you’ve been lost for weeks in a desert. Even doing a simple physical task in the summer sun can lead to a drop in attention, coordination and complex problem-solving abilities, though you’ll hold onto your normal reflexes. (So they can save you from the consequences of all your other sudden deficits?)

In trying to find out just how much loss of the body’s water led to a chink in the cognitive armor, researchers analyzed 6,591 studies to compare the results. They focused on acute dehydration (a more common kind of dehydration experienced from heat or exertion, unlike the more extreme chronic dehydration, caused by a disease or disorder), and found that a loss of water equal to 2% body mass is when severe physical and cognitive impairment sets in.

“There’s already a lot of quantitative documentation that if you lose 2% in water it affects physical abilities like muscle endurance or sports tasks and your ability to regulate your body temperature,” says Mindy Millard-Stafford, a professor in Georgia Tech’s School of Biological Sciences. “We wanted to see if that was similar for cognitive function.”

It was. Researchers found that as participants became increasingly dehydrated, they made more mistakes when it came to attention-related tasks – repetitive and dull tasks, such as pushing a button in different patterns for a while. Similarly, in daily activities, dehydration can lead to errors in judgment.

“Maintaining focus in a long meeting, driving a car, a monotonous job in a hot factory that requires you to stay alert are some of them,” explains Millard-Stafford. “Higher-order functions like doing math or applying logic also dropped off.”

While it might not seem like much, a 2% drop in the body mass of a person weighing 90 kg would mean a drop of 1.8 kg of body weight. And it can happen perhaps more quickly than you’d think.

“If you do 12-hour fluid restriction, nothing by mouth, for medical tests, you’ll go down about 1.5%,” she says. “Twenty-four hours fluid restriction takes most people about 3%.” And that’s when the cognitive and physical effects of dehydration take off. For older people or obese people, the dangers of dehydration are felt even more, as their bodies lose water more quickly.

But even though “water is the most important nutrient,” the solution isn’t to swill it non-stop, Millard-Stafford says. If you’re an active person, or spend a lot of time without A/C in a hot climate, you should take pains to stay hydrated, as well as to consume salt, which helps you actually retain the water you drink. If you’re more sedentary, or live in cooler climes, don’t, er, ‘sweat it’ as much.

“You can have too much water, something called hyponatremia,” explains Millard-Stafford. “Some people overly aggressively [sic], out of a fear of dehydration, drink so much water that they dilute their blood and their brain swells.”

“Water needs to be enough, just right,” she says.