Interesting that you post this, because it is one of the things I am examining with complete focus right now. If you develope a "bad taste" in your mouth shortly after falling asleep, and get itchy scalp\bottom , then you fall into the " parasite infection" catagory {just like me!}. There may be slight or heavy coating on tongue, usually white. Sometimes small patches will develope. This depends on bacteria present. If you have a bad tooth, for example, then this can be a factor.
The bad taste is from high levels of nitrate\nitrite in mucus part of the saliva. This has a detrimental effect on certain immune-functions, but is very anti-microbial at the same time. It is essentially poison at these levels.
When you are really sick, like with flue, you may have a bad taste in your mouth. This could be due to high levels of this chemical {postulation}.
High levels of nitrite being swallowed can cause smooth muscle contraction resulting in all sorts of chronic health problems too detailed to go into. Shroom