Unfortunately that doesn't seem to work for me. Right now my room has > 1200ppm CO2 (due to closed doors/windows) but I don't feel sleepy at all, or any differently (that I can discern) from when it's < 600ppm. Do you know if it needs to be significantly higher for that feeling to kick in? If so, approximately how much?

Have a meter going and look at it. I’d describe it as a feeling in my lungs as i breathe I can start to feel above 800ppm. The air feels heavy maybe?

I think some people are more sensitive to it than others.

Have you tested yourself by concentrating on your breathing a bit and then guessing the level before you look at it?

I'm not disbelieving, just curious.

Hmm, I've never had the device to measure the levels tbh. I can say I'm fairly sensitive to air as i do breathwork regularly. 1200 ppm sounds a lot, maybe that's coming from not just CO2 in the air but maybe dust particles, smoke, food vapour, or chemicals such as odor, ozone, perfume, house cleaning products etc.

    Unfortunately that doesn't seem to work for me.

    Right now my room has > 1200ppm CO2 (due to closed 
    doors/windows) but I don't feel sleepy at all

It's a very gradual difference. If you're doing something interesting at 1000-2000ppm you probably won't feel sleepy, but it will probably be harder to focus on something you don't find stimulating and you'll be more inclined to feel sleepy.

Interesting, I guess I can try to see if I can notice that. Thanks!

I always assumed that this was somewhat natural, but it may just be my upbringing. I grew up in a warm country without air conditioning, so open windows were the only way. Then moved to a cold country and noticed the sensation. I put 1-1 together.

In my case my face, specifically my forehead, feels warm. Not to the touch, but more to the nerves. I think it may be different for different people.