Dehydration is often associated with feeling light-headed. Long periods in the heat and sun, especially after exercising, can leave a person feeling dizzy and in need of water. Sometimes feeling light-headed can be caused not by a lack of water, but by drinking water.
Water is crucial for health. Factors that increase the need for water include environment, exercise, illnesses and pregnancy. Lack of water, or dehydration, can cause many health risks aside from feeling faint and dizzy.
While not common, it is possible to drink too much water. This can strain the kidneys as they attempt to rid the body of the liquid. Too much water in the body dilutes the sodium levels and dizziness and light-headedness can occur.
When a large amount of water is consumed in a short period of time, the brain may malfunction due to an electrolyte imbalance in the body's fluids. This is called water poisoning and feeling faint, dizzy, and nausea may occur.
According to St. Mary's Hospital nutritionist Tammy Fumusa, a person will most likely avoid feeling faint and dizzy from drinking too much water by simply going to the bathroom.
Runners need to drink water after they run, but too much water after a race can disturb electrolytes and put pressure on the heart. A Boston Marathon runner in 2008 died from a heart attack after drinking too much water too quickly.
Water is not toxic or poisonous but rapid and large amounts can be dangerous and disrupt sodium levels and electrolytes and tax the body's organs. A 2008 radio contestant died after drinking too much water without being able to use the restroom.