system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we drool when we sleep? More specifically during daytime naps...
gpt
Drool isn't produced when you're completely asleep... however, it is stimulated by the parasympathetic nervous system (also known as the "rest and digest" half of the nervous system, the calming half).

One of the main reasons for drool is to keep your mouth wet, your airway lubricated, and components in it have a germicidal effect, killing off some of the bad bugs that like to colonize your mouth.  

When you're falling asleep, these factors are still in play - and also, due to your horizontal position, less saliva goes down your throat.  The combination of the parasympathetic nervous system being stimulated, the change in posture, and the general relaxation of muscles.

Once asleep, however, the drooling stops - which is why your mouth tastes bad when you wake up!  Your mouth has dried out and new bacterial colonies have popped up, leading to a nasty taste/smell.  

IF you are still drooling while fully asleep, this can point to underlying issues in sleep, such as sleep apnea.