I have never bothered to measure the CO2 levels in my house, but I have made sure to have fresh air from outside.
I don't have a thermostat or ventilation in the bedroom, but the ritual of cracking open a window, even just a touch in winter, is for me an essential part of creating a suitable environment for sleep.

But besides clean air, I've also invested some money in creating a comfortable sleeping environment. I've been sleeping on a Pluto pillow, which has truly upped my comfort, and a Leesa Studio mattress, which is great for all sleep positions.

To top it all, I've gone for minimal lighting, and my bedroom is as tech-free as possible. No TV, no laptops. Just a simple digital clock.