I was suspicious about this after noticing that whenever my son had a nightmare and I slept in his room, I'd end up in a really bad mood in the morning, moreso than hours of sleep alone would predict. Bought a CO2 monitor, and ... yup.
I slept with a window open last night after seeing this convo.
I do feel more clear headed. I also had more dreams. However: my lips are chapped, throat is scratchy, and my jaw is very tired/tight (I grind my teeth when I sleep cold, so that's my explanation). Unclear if net +