Livestrong says:

You’re feeling fatigue, have a headache, or are just plain old grumpy: In a study published in The Journal of Nutrition, participants who were dehydrated by more than 1 percent reported decreased mood, lower concentration, and headaches. According to the study’s authors, certain neurons detect dehydration and may signal other brain regions that regulate mood and cognitive functions. “A rule of thumb is that women need about 11 8 oz. cups of water a day and men need around 15 cups,” says nutritionist Amanda Berhaupt-Glickstein.

>>You want to lose weight: In meeting of The American Chemical Society, researchers found that over 12 weeks, dieters who drank water before meals three times per day lost about 5 pounds more than dieters who did not increase their water intake. Good guidelines: Drink two cups before every meal.

>>You exercise for less than 90 minutes: Just because you sweat doesn’t mean you should reach for a sports drink. Yes, you need water for rehydration and because it helps lubricates joints and provides cushioning to organs and muscles, along with many other vital processes. However, people often overestimate their needs for sugar and sports drinks when exercising, says Lisa C. Cohn, owner of Park Avenue Nutrition in New York. “Really, only water is needed unless you are active for more than 90 minutes with moderate to high intensity.” Drink about 15 to 20 ounces two to three hours before exercise, and 8 to 10 ounces 10 to 15 minutes beforehand, and the same amount every 10 to 15 minutes during exercise.” Get the rest here: What To Drink (and When To Drink It) | LIVESTRONG.COM.