Have you ever noticed that the water tastes different when you leave it in a glass on your nightstand overnight?
In general during the 8h sleep, CO2 is built inside the room. Especially in winter when we don’t ventilate as frequent and CO2 can reach levels higher than 2,000ppm inside the house. CO2 is a soluble gas and water is the perfect solvent. As a result a process called Acidification is occurred naturally and the PH lowers during that process. That gives your bedside glass of water a horrible taste.
See The Air 