Lack of water slows everything in the body down, leaving you feeling sleepy and tired. Adequate hydration is dependent on many different factors, such as your size, activity levels and sweat rate. “By the time you’re thirsty you’re already dehydrated,” Dr. Roberts says, “but, in general, drinking enough water when you feel you should is safe enough.” The best indicator of your hydration is your urine. Someone who is well-hydrated will produce clear or light-colored urine. Dark-colored urine is a sign of dehydration.