Most get used to taking a glass of water by night beside them, on the bedside table if they are thirsty. But you will have noticed if you do that if you drink it the next day, its taste is slightly altered. What has happened;
Is there a glass of water that has been left all night on your bedside? It is true that overnight there are chemical changes that can change the composition of water, making it slightly acidic. Also, since water does not contain proteins and sugars, it is easier to be exposed to germs, as opposed to eating.
So when you leave a glass of water exposed to the atmosphere for hours, it absorbs carbon dioxide, reduces its PH and becomes acidic. This process does not make it dangerous, it just changes its chemical composition and thus its taste. It is actually very easy, a glass of water that has been left long hours out, exposed to bacteria and has enough dust.