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I did quite a lot of research trying to find the best CO2 sensor that would also do humidity/temperature. CaDi by nuwave* is what I settled on. An Irish company. I've been using it for a week now and it is working great! Things that surprised me: the difference it makes if I leave all the internal doors open in the apartment, not just the bedroom door. That even leaving one window open just a crack will help with ventilation. And of course how quickly a room with couple of people in it can go above 3000ppm CO2.

*https://nuwavesensors.com/product/cadi-ci-100/

I always sleep with the window open, because the fresh air feels nice.

In evolutionary terms, you could speculate that the nice feeling of fresh air could be an instinctual response directing the organism to seek higher quality air.

I don't have any domain knowledge on CO2 monitors, but I use this one: https://smile.amazon.com/gp/product/B01FYWU2IS/

It's a greenhouse CO2 monitor, which is the most common kind. I've heard that they're not as accurate as CO2 monitors that are designed for humans, but those tend to be much more expensive.