It doesn't actually have to be the CO2 per see. It can be used to measure overall air and ventilation. Or how many humans there are in a house/room, and if they are asleep or awake. The drowsiness is probably due to both low oxygen levels and high CO2 levels, and more.
Come on, haven't you heard of photosynthesis, oxygen, and pollution before? Higher than average ambient CO2 levels is basically like sitting in an oxygen deprived chamber. Also, there is a recreational drug called Carbogen that is a mixture of Carbon Dioxide and Oxygen that, although actually fine for breathing biophysically, has a reduced level of oxygen and increased level of CO2 that makes your body believe it's dying. Very interesting. So, even if it's not a lethal difference in composition, higher CO2 levels will definitely affect one's mood adversely.
