If you have kept water in your glass from yesterday, in the morning after waking up, you should not drink it, instead pour it immediately and do favor to yourself.
The ordinary water unlike the food, is not containing sugars or proteins so, this is why the microbes can be easily contaminated. For the strange taste that is in the water that was left during the night there is an explanation. Namely, when the water was exposed to the air, it was in touch with CO2 and part of this CO2 is converted to carbonic acid which means that the water which is exposed to air changes its chemical composition. When the carbon acid will dispose one or two protons and will turn into bicarbonate or carbonate, it reduces the pH inside the water and therefore changes the taste.
The next time you reach for glass of water from the previous night; remember that it was exposed to bacteria that inside there were present large amount of dust. That does not mean that the water is tainted, but because of the things that happened during the night do not drink it.