system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens while we are asleep that makes our breath smell so bad when we wake up?
gpt
You're not moving your mouth around as much, and in some cases you leave your mouth hanging open all night which dries it out.

The smell of bad breath is the smell of ~~your saliva~~ bacteria breaking down food in your mouth - called putrefaction.  During the day you talk, yawn, drink, eat, smoke, run, and salivate and swallow.  The process continually exposes and flushes (by way of swallowing) the decaying food.  

When you sleep, you mostly lay there.  The saliva pools and chemically digests food, and the results stick to your tongue, teeth and gums.  This stuff stinks.

If you take a decent solo road trip, like four plus hours, and stay quiet the whole time, your breath will usually stink by the end.