Drooling during sleep is usually less dramatic than it looks.
Your mouth is still producing saliva.
You are simply swallowing less often and controlling the lips and jaw less actively than when you are awake.
Saliva production continues during sleep
Your salivary glands do not completely shut down at night.
Saliva still helps protect the mouth and tissues.
But the swallowing reflex happens less often during sleep.
That means saliva can collect more easily.
Relaxed muscles matter
Sleep reduces muscle tone.
The jaw may relax.
The lips may part.
If you are sleeping on your side or stomach, gravity can then give saliva a convenient exit route.
Why do some people drool more when congested?
If your nose is blocked, you may breathe through your mouth.
That makes an open-mouth sleeping position more likely.
This connects directly with why your nose gets stuffy.
Why does sleep position matter?
Sleeping on your back makes it easier for saliva to stay in the mouth and move toward the throat.
Sleeping on your side or stomach gives gravity more opportunity to pull it outward.
That does not mean everybody who sleeps on their side drools.
It simply changes the odds.
Is drooling always normal?
Occasional sleep drooling is common.
Heavy new drooling, trouble swallowing, facial weakness, or other neurological symptoms are different and deserve medical attention.
For most people, though, the explanation is much simpler.
Why can drooling and dry mouth both happen?
They sound opposite, but both can happen.
If you sleep with your mouth open, saliva can escape while airflow also dries the remaining mouth surfaces.
That is one reason you can wake up with a dry mouth after apparently drooling half the night.
Your saliva production did not suddenly go wild.
Your sleeping mouth just became a less reliable container.
And that’s the short explanation.
Related explanations: Why do we wake up with dry mouth?, Why does your nose get stuffy?, and Why do we snore?