Large number of people have the habit to drink water in the morning, right after they wake up. Probably your parents have told you that it is very healthy.
If you have this strange habit, probably you have noticed that the water have strange taste.
The food contains sugars and proteins which cans be easily targeted by microbes and to come to pollution. Although regular water does not contain any of these nutrients and there is no explanation from which comes the strange taste in the water that stood on the nightstand over the night by your bed.
Still, there is large amount of dust and bacteria that can be gathered in that glass of water. When water is exposed to air it absorbs carbon dioxide and a small part of that CO2 is converted into carbonic acid, which means that water exposed to the air changes its chemical composition. So, when carbon acid releases one or two protons and is turned into carbonate or bicarbonate, it lowers the pH of water and changes its taste.
The water actually is not bad, but certainly has changed the taste after standing all night in the nightstand and gathering bacteria and dust from the air.