On the other hand, water left in a glass is more susceptible to dust particles and air-borne bacteria.
On the other hand, when water is exposed to air, it absorbs carbon dioxide, a small part of which is converted into carbonic acid. In other words, this implies that water exposed to air is prone to changing its chemical structure. To be more specific, carbon acid converts into carbonate or bicarbonate when it releases one or two protons, which in turn changes the water pH. This change also affects its taste.
Although this doesn’t necessarily mean that water that’s been on your nightstand all night is harmful to drink, it’s definitely something worth considering due to the fact that this water has collected a lot of dust and air-borne bacteria. So, the next you feel thirsty upon waking up, simply hop to your kitchen and get some fresh water instead.