Water is absolutely essential to good health. It accounts for about 60% of the average person's body weight and is crucial to body functions, such as flushing out toxins and carrying nutrients to cells. While everyone needs water, it's often hard to know how much one should drink. According to Dr. Andrew Yacht, Vice Chair of Medicine at Maimonides Medical Center, "With a normally functioning heart and kidneys, and under normal physiologic circumstances, eight glasses per day is appropriate." Slightly more or less water may be called for depending on your level of physical activity, sweating and frequency of bowel movements. Dr. Yacht stresses that, "You want to avoid dehydration, even a mild case of which can drain energy and make you feel tired. Less common, but equally important, is chronically drinking too much (more than a few liters per day). Drinking too much water can cause your blood sodium level can drop creating a potentially dangerous condition called hyponatremia, which can lead to complications such as neurological issues."