system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5- Why does my mouth taste all gross and icky when I wake up after 6-8 hours, even if I brush before bed, but it doesn't feel that gross if I'm awake without brushing for much longer?
gpt
Dentist here...

It's because your mouth IS gross after 6-8 hours of sleeping.  When you are awake, your saliva production is increased, your tongue, lips, and cheeks push that saliva around in your mouth, and it all gets swallowed.  That saliva washes away food, bacteria, fungi, viruses, dust, hair, bugs, whatever down into your stomach where it is digested.  Talking, chewing, licking your teeth all help the process along.

At night, all of that stops.  You produce very little saliva (yes, you may drool a little, but it is a very small amount over those 6-8 hours compared to day time saliva production).  Your tongue is very still, your lips and cheeks also stop moving for very long periods of time.  Swallowing also is reduced, and many people sleep with their mouths open, which allows for ample accumulation of dust (dust is very gross), mites, pollen, hair, bacteria, yeast, etc to collect in your warm, moist incubator-like mouth.  

After several hours, your saliva in your mouth begins to evaporate, leaving a thick, sticky film laden with gross and icky stuff.  That bacteria that naturally occurs in your mouth and the air around you has had countless numbers of reproductive cycles, and now your mouth is one giant petri dish.

LPT: practice sleeping with tape over your mouth.  Nasal breathing is much healthier and will help eliminate some of your nasty-morning-mouth issues.