Mild dehydration can occur with as little as a 1.5 percent loss of fluids in the body. This can occur from illness but simply skipping drinking a few glasses of water for a few hours can have an effect on a person and how they feel. Once a person is two percent dehydrated they may start to have a noticeable difference in how they are feeling or acting. Dehydration can affect anyone which is why it is important to drink at least eight glasses of water each day or more depending on the weather and your activity level. If you have just exercised you will want to drink down some water (Electrolytes help as well) and if you have been outside in the heat you will want to over compensate for that as well.