I find it especially a problem at night. Started out leaving the window open in the bedroom, but that lets in noise and wind. Current solution is leave the bedroom door open and have a window cracked open in another room. It seems to work out really well as measured by my CO2 monitor.

I mentioned this elsewhere here, but a friend told me that it’s not only CO2 that leads to that mental fog, but it’s also the VOCs in the air too. All that gets cleared out by having an open window.