It’s normal to feel thirsty after eating spicy foods or doing strenuous exercises. But sometimes thirst has a hidden reason. You may have heard of the “drink eight glasses of water a day” standard rule. Even though it’s not the best basis for measuring proper hydration, many people follow it and are healthy. Adequate hydration is dependent on many different factors, such as your size, activity levels and sweat rate, but certain conditions have a tremendous effect.