Factors of drooling while sleeping

I didn't begin drooling while sleeping until I hit the 60s. I think it's mainly due to my mouth being open. I don't know why my mouth opens now when it never used to. I can prevent it from happening by closing my lips, putting my tongue against them and sucking a vacuum inside my mouth to hold them closed. That is, if I remember to do it. You don't swallow while asleep and it seems that we stop producing saliva also, so it's only necessary to keep the lips closed for a short while after getting to sleep.

I wake up in the morning with a dry mouth whenever I sleep with with my mouth open. This usually happens when I have a cold and I breath through my mouth during the night. The drooling only seems to occur as I'm first going to sleep.

Why is saliva coming out during sleep? How to get rid of this habit?

Why does saliva come out while sleeping, and also while I wake up I get full mouth of saliva?