Back when CO2 was ~300ppm in the atmosphere I used to sleep with windows closed. These days now that it's well up over 400ppm I'm putting fans in front of windows.

It makes sense since a doubling (from being indoors) of 300ppm isn't pushing the noticeable 800ppm range whereas a doubling of 420ppm is.

These recommendations probably worked in the 80s in my experience.