I went out for pizza on Saturday night and, despite the thousand-plus times I told myself "Don't eat five slices," I ate five slices. I don't do well with that much sodium (and also don't learn from my previous mistakes), so I put a ginormous glass of water beside the bed so I'd be prepared when I inevitably woke up at 5 a.m., feeling like there was an abandoned pepperoni factory on the back of my tongue.
But instead of crisp, cool refreshment, that water tasted like ... dust? Stagnation? What even is that flavor?
We've all gotten a mouthful of weird-tasting morning-after water, but why does the flavor change? And is it safe to drink? The short answer is that it's perfectly fine to drink. Let me explain why.
What happens to water when it's left out overnight?
There are a couple of theories on what happens to a glass of water when it's left out overnight. The first is that, if it's tap water, the chlorine that was probably added to your municipal water supply has evaporated during those hours, which can change the flavor. (Wired reminded us that this is exactly why you need to let tap water sit before you pour it into your goldfish tank). And regardless of the water source, after being left out for several hours, carbon dioxide can begin to react with the water, which lowers the pH a tiny amount — just enough to change the taste.
During a surprisingly in-depth discussion on Reddit, a food chemist pointed out that taste is one of the most complicated senses, suggesting that we might be comparing that "stale" water to the memory of how it tasted several hours ago. He also said that the water will have evaporated slightly overnight, which can concentrate the flavor of the "other stuff" in each glass. "The most likely answer you're looking for," he wrote. "You are tasting/smelling the air in the glass, rather than the water. This air is a concentrated version of any volatile molecules that [have] off-gassed overnight, [and] likely didn't make it up the sides of the glass and out of the system due to air pressure."
Fine, but is it safe to drink?
As far as the safety issue, Munchies points out that water doesn't have any "sugars, fats, or proteins that can rot or go rancid," so it didn't "go bad" after several hours, despite the weird flavor. The caveats to that seem to be ensuring that you don't leave the water out for days or weeks, don't reuse the same glass forever without washing it, and don't share that same unwashed glass with other people.
Also, you should wash your hands after using the bathroom so none of those microorganisms or bacteria (we don't need to go into the details) are transferred to the water, but that's a solid practice regardless of how long that glass has been sitting on your nightstand.
Do you notice that a glass of water tastes funny when you leave it out overnight?