Have you ever woke up to find that your glass of water has acquired an odd flavor, like a stale or even earthy taste? You’re not alone. As it turns out, the icky-tasting hydration moment you’re experiencing is not in your head—subtle chemical changes happen overnight that can actually change the composition of your water, making it slightly acidic.
According to an April 2016 episode of the SciShow podcast, a lot goes into your water that doesn’t necessarily affect your taste buds. Because it’s a solvent, your water can dissolve other molecules and ions into it, including gasses like carbon dioxide—which is emitted during our breath throughout the night. This process forms carbonic acid, which makes your water a little more acidic.
When the temperature of your water reaches room temperature, its solubility increases, meaning gases are more likely to disperse and react with it. As a result, your water can start to taste strange in the morning, especially if you have been urinating frequently during the night.
If your water has an earthy or musty taste, it may be the result of algae blooms in your local water source. Waterlogic notes that a taste like this could also be caused by metal plumbing or bacteria, as well as treatment chemicals or organic material in your water supply.
Fortunately, your water is still safe to drink when it tastes off in the morning. If you’re concerned, try putting a lid on your water bottle or using a closed water bottle instead.
