Everyone gets unhealthy CO2 levels if the bedroom doesn't have air extraction inside or you don't leave the door opened.
Does it really matter in the grand scheme of things tho ? I have a captor at home, even when I leave the door opened and CO2 remains low, I don't notice anything different at wakeup.
It can be the difference between between feeling like you're suffocating, not getting enough oxygen to rest enough/sleep well.
I notice a difference if I move between a ventilated room vs congested one. I suppose it depends on what's causing the concentration. If it's human breath, I'll smell freshness. If it's e.g. burning a portable gas heater (common in my part of the world), I'll feel like I'm not inhaling smoke (probably small amounts of CO).
A few years ago, I would sometimes wake up at night and open a window wide, or go open the outside door and stand there for 5-10 minutes.