Drooling is when there is excess saliva that comes out of a person’s mouth and we all do this every once in a while. But did you know that when this phenomenon occurs regularly and excessively, it can be a sign of a developing disease or some malfunction taking place in the body?
When we sleep our facial muscles, as well as our swallowing reflexes, are totally relaxed. Since saliva gets accumulated in the mouth while we sleep, it can slowly start dripping because the relaxed facial muscles may lead to a slightly open mouth. Therefore, we end up having a wet pillow that’s not very comfortable to sleep on.
Excess drooling or hypersalivation can be a sign of a neurological disease or the result of nasal congestion. Also, people who already had issues with health, like a stroke, tend to drool more often and more excessively.

You drool when you don't close your mouth while sleeping, it makes your pillow and bed smell.
Close your mouth while sleeping. Not everyone drools while they sleep.