Do you leave a glass of water next to your bed in case you get thirsty at night?
Apparently it's not a great idea and not the safest thing to do.
With an open glass of water, dust and bacteria can fall in and create a film of bacteria on top.
Even a closed bottle might not be a good idea because once you take a drink, bacteria from your mouth and lips gets in there and starts multiplying.
So listen to the experts advice: once you've put your lips to a bottle of water, you should try to drink it the same day or dump it out.