Damn,
When my son was about 10 months old til he was about three my wife would go in his room with a mirror and put it under his nose .
I’d say what the heck?
Well he just slept so soundly she was worried.
Then he broke his nose in football and snores.
Thank goodness.
As a child I slept very soundly. So did my kids.
I used to fall out of bed on a wooden floor just about every night. My grandfather would pick me up and put me back in the bed. The next morning I didn’t even know it had happened.
A freight train could have gone through our home and my kids wouldn’t have heard it.
I once got my daughter up to go to the bathroom before I went to bed. A few minutes later I checked on her. She was standing in the middle of the bathroom floor dead asleep.
I never thought of using a mirror, but your mention sure does bring back memories.