I remember seeing a video many years ago which, microscopically, looked at the surface of water that has stood overnight. It was covered in dust mite faeces, material fluff, dander from pets (if you have them), tiny bits of cobwebs, and other sources of dust (including things that had come in through the window); everything that's floating in the air settles on it overnight (think about what happens when you dust a piece of bedroom furniture - what's on the cloth is what's going in the water). After that, I've never kept a glass of water by the side of the bed without covering it (only in case I need it in the night). And, if I get through the night without needing it, it goes straight back to the kitchen and down the sink. Then I have a glass of FRESH water.