By the time a person feels thirsty, dehydration has already kicked in. This not only means that the urge to quench thirst is strong, but unfortunately, that the body's mood, mental ability and energy levels have already been compromised. "Our thirst sensation doesn't really appear until we are 1 or 2 percent dehydrated. By then dehydration is already setting in and starting to impact how our mind and body perform," says Lawrence E. Armstrong, professor of physiology at the University of Connecticut. Armstrong was also the lead researcher in a study on the subject of hydration, which reinforced the importance of drinking plenty of water.