If you have a glass of water in the evening, will you drink the remainder in the morning? I do. All of the time. It never once occurred to me to even question the safety of day old water until I saw an article on treehugger questioning just that. I mean, who knew this was even an issue to worry about?
Turns out, it is pretty safe…overall. It begins to get an off-flavor because of the carbon dioxide that changes the pH balance in tap water. Plus, microorganisms start to grow as soon as you take a sip and introduce your saliva into the mix. Over the course of a couple of days, neither is likely to cause you much trouble health-wise. It only takes about 12 hours before the taste is affected, though.
I’m still in the camp of waste not, want not. I have been drinking little microorganisms this long without trouble, so I am going to press on. But seriously, have you ever even given it a second thought before now? There’s a ridiculous amount of things to get worried about and worked up over if you let yourself, eh?