It quickly confirmed that, without turning my thermostat fan to "ON", my bedroom CO2 jumps from about 1000 ppm to 2000 ppm within a couple hours of us going to sleep.
In the fall, my wife and I started waking up in the middle of the night feeling like we couldn't breathe. What I figured out is that the neither the heat nor cooling was running because the weather was pretty much perfect. We have to keep the door closed because our cat will swat at us in the middle of the night otherwise. I figured there would naturally be enough of a draft to not make this a concern and it's a fairly large connected master bedroom/bathroom with high ceilings to hold a lot of air, but I guess that isn't enough. So that's when I learned about the "fan" function on my thermostat.... never had a need for it in my life.
Now I just need to figure out how to get lower than 1000 ppm, which is already a hit against cognitive function. Our house has no screens currently and even if we install some, it's not like I want to have windows open in the middle of winter. I've heard house plants don't have a huge effect, but I'm willing to try.