It's true that your body needs water to function properly, but your body wasn't designed to get all of the water you need for a whole day at once.
Drinking too much water can cause a life-threatening condition called hyponatremia, which is sometimes called water intoxication. Hyponatremia means "low sodium in the blood." It occurs when someone drinks enormous amounts of water and/or loses too much salt from the body in a short time.
A large intake of water overwhelms your kidneys so they can't process and eliminate the water fast enough, and the levels of sodium in your blood can get too low. Severe sodium deficiency can lead to twitching, seizures and even death.
Hyponatremia can happen when marathon runners drink gallons of water and don't replace the electrolytes during a race, or when people with certain psychological disorders can't stop themselves from drinking water. It can also occur in older people and can happen with certain medical conditions.
A person with hyponatremia needs to see a doctor for immediate diagnosis and treatment.
Drink the water your body needs, but don't drink more. Let your thirst and the color of your urine be your guide. If your urine is pale yellow, you're just fine.
Don't drink more than 30 ounces per hour if your'e exercising intensely -- like running a marathon.
Drink sports drinks with electrolytes, they'll help keep your potassium and sodium levels about where they need to be.
Speak with your doctor if you have a medical condition that involved your kidneys or your adrenal glands, or if you take an antidiuretic.
American Family Physician. "Management of Hyponatremia." Accessed February 9, 2012. http://www.aafp.org/afp/2004/0515/p2387.html.