We all know that water is good for us and it's important to drink enough of it, especially first thing in the morning to jumpstart your metabolism and reset your digestion system. However, many people find that their water tastes strange in the morning - and it's not just in their head. Apparently, there are some subtle chemical changes that happen over the night that make your water taste different. SciShow explains the reasons why this happens, including that water can be contaminated by things that leach into it from the pipes.
In addition, when you sleep, saliva secretions dry out and leave your mouth in an acidic environment, making the water you drink taste strange in the morning. Other conditions like chronic acid reflux can also cause this, as stomach acid regurgitates into your throat and mouth overnight, causing a sour or acidic taste to the water you drink in the morning.
Other factors that could influence the way your water tastes include whether it's spring or well water, and how the water is treated. For example, spring water often comes from a mountainous region with a lot of clean runoff from snow and rain. It's often filtered, but mineral deposits from the soil can still affect the taste. Well water, on the other hand, comes from underground aquifers that are filled with various metals such as iron and copper. These metals may have a metallic flavor that can be enhanced when the water is hot.