Dehydration will manifest as fatigue, and when people are tired, they are usually in a worse mood. There are scientific reasons as well. Even mild dehydration – about 1.5 percent loss in normal water volume in the body – can alter a person’s mood, energy level, and ability to think clearly, according to two studies conducted at the University of Connecticut. Neurons in the brain detect dehydration and may signal other parts of the brain regulating mood when dehydration occurs.