[selfie]

I assume you are not aware of it, because of being asleep. Maybe it happens in deeper sleep?

[MichaelRo]

Could be but I'm only talking of what I can consciously control. I do know I snore if I sleep on my back, that's probably sleeping with the mouth open.

[TheCapeGreek]

The tongue and jaw muscles slack and relax when you fall asleep, like much of the rest of your body. For many, this causes apnea and heavy snoring.