You may think it’s good to glug loads of water during spin class — or that it can’t hurt. But there is such a thing as overhydration. Hyponatremia, in which the level of sodium in your blood gets too low, can be caused by drinking large volumes of fluid, even with electrolytes. It’s rare, but it can be deadly. “There is no reason to drink more than your body needs, and the sensor that tells us how much we need is thirst,” says Dr. Mitchell Rosner, professor of medicine at UVA Health.