There are several ways to determine if you're not getting enough water. Clear indicators include feelings of tiredness, fatigue, irritability, or constipation. Dizziness, particularly what's referred to as postural dizziness (when you stand up and instantly feel lightheaded or your heart races), can also signal dehydration. Another sign is an unexplained increase in your pulse rate. This occurs when you're dehydrated and your brain is trying to regulate blood flow to the heart.