Waking up with a wet pillow can be embarrassing, but drooling during sleep is actually common. It happens when saliva escapes from the mouth while you are sleeping. In most cases, occasional drooling is harmless and does not mean that something is seriously wrong.
One common reason is sleeping position. People who sleep on their side or stomach may be more likely to drool because saliva can flow out of the mouth more easily than when sleeping on the back.
Nasal congestion can also contribute. When your nose is blocked because of allergies, a cold, or sinus problems, you may breathe through your mouth while sleeping. This can make saliva escape more easily.
Another possible cause is deep or relaxed sleep. During sleep, the muscles around the mouth and throat relax. Swallowing also happens less frequently, allowing saliva to collect in the mouth.:.Read the full now & the viral video.