While research shows that just bringing a bottle of water with you to a test can improve your scores, a lack of water leaves your brain parched — bad news for cognitive performance. A glance at the studies reveals the many ways dehydration hampers our mental lives: Mental calculation abilities go down. Mood swings go up. Irritability increases. Fatigue increases. Short-term and long-term memory are impaired. It's all because our brain cells need water to function. "Brain cells require a delicate balance between water and various elements to operate," says University of Texas neuroscientist Joshua Gowin. "When you lose too much water, that balance is disrupted. Your brain cells lose efficiency." Beyond getting enough sleep and eating healthy food throughout the day, staying hydrated is one of the best ways to stave off performance-eroding fatigue. There are lots of ways to become dehydrated. Marinating in the summer heat and sweating through a morning workout are obvious places where we lose water, but nutritionists say that dehydration isn't just a matter of athletics. You'll lose water throughout the day just being in the office, and Gowin notes that the longest we go without fluid intake is the six to eight hours we're sleeping — meaning we're dried out by the time we wake up. What's important is to not wait until you're thirsty, since the feeling of thirst doesn't show up until you've lost 1% to 2% of the water volume in your body. "By then dehydration is already setting in and starting to impact how our mind and body perform,” says Lawrence E. Armstrong, professor at the University of Connecticut's Human Performance Laboratory. "Dehydration affects all people, and staying properly hydrated is just as important for those who work all day at a computer as it is for marathon runners, who can lose up to 8% of their body weight as water when they compete." Naturally, the opposite of dehydration is hydration. It's hard to say what the "right" amount of water to drink every day is, since it's dependent on factors like your age, climate, and level of physical activity. But the consensus is around 1 to 2 liters a day, or about 6 to 8 glasses.