Research has already shown us that enduring a heat wave without air conditioning can impair our mental performance. Now, a new paper reported by NPR shows that not drinking enough water in the summertime can have a similar impact. According to a meta-analysis published in July in the journal Medicine & Science in Sports & Exercise, even mild dehydration can negatively affect cognition.

Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Institute of Technology, looked at 33 studies dealing with dehydration. She found that subjects performed worse in a range of areas—including attention, motor coordination, and goal-oriented thinking—when they were slightly dehydrated compared to when they had consumed enough water that day.

She defined mild dehydration as fluid loss equivalent to about 2 percent of body mass. On a hot summer day, reaching this stage can happen surprisingly fast. If you're hiking in the heat, it might take you an hour to become mildly dehydrated, and if you're going on an intense run, it can take just 30 minutes to sweat out 2 percent of your mass.

That level of dehydration isn't too noticeable (you may just start to feel thirsty), but if you have to do something that requires your full attention, it makes a big difference. In one study published last year, female subjects who were 1 percent dehydrated made 12 percent more errors while playing a game that required them to think quickly. (That paper was funded by PepsiCo, which sells bottled-water brands like Aquafina and Propel, but the researchers designed the study independently, according to NPR.)

Luckily, the brain fog that comes with dehydration has a simple antidote: a glass of water. The biggest roadblock keeping you from staying hydrated may be your inability to recognize dehydration in the first place. To see if you're getting enough water, use this easy skin test throughout the day.

[h/t NPR]