It’s because you didn’t actually remove all the bacteria in your mouth during your last brushing. I’m not insulting your brushing habits, it’s just not realistic to expect that you removed 100% of the bacteria every single time you brush. Also, remember that bacteria and other pathogens are airborne. This means even if you did succeed in removing every single last bacterium when you brushed, your first breath contaminates your mouth all over again. Essentially we are playing catchup at all times, trying to keep the inevitable invasion of bacteria at bay as best as we can.
When you sleep at night, you don’t produce as much saliva (spit) as when you are awake. Saliva helps protect and wash your teeth. The reduced production allows the bacteria inside your mouth to multiply overnight, causing the terrible taste and bad breath. If you sleep/breathe with your mouth open, your teeth dry even faster, worsening the effect.
This is also why you are more likely to drool when napping because your saliva production doesn’t go down as much as when you are asleep at night.
The bacteria had time to multiply and grow, but the biggest reason is that your mouth dries out while you sleep (especially if you breathe through your mouth at night, as you probably do if e.g. you’re overweight). That drying concentrates the sugars and proteins dissolved in your saliva into a thick, sticky film that (a) feels and tastes kinda gross and (b) is a lovely place for bacteria to hide out.
In general, it’s because your mouth produces less saliva while you sleep. That slightly drier and more importantly less enzymatic environment along with a long period without any rinse (like drinking water during the day would provide) makes a very hospitable environment for bacteria to reproduce. That bacteria feeds on food particles and sugars and produces acids which can harm your tooth enamel, but also produce a strong smell (halitosis). So when you wake up, you’ve got lousy breath.
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