I notice this the most in cars, when I set the air con to recycling air (instead of pulling in fresh air). After a while, I start to feel sleepy, and the sleepiness passes almost as soon as I open a window or change the air con setting to bring in fresh air.
I notice it also in shopping centers, or long bus rides, where there is inadequate fresh air. I know a lot of people who get sleepy from this, but also some people who just notice the air smelling 'stuffy' - but there's no particular smell, just 'stuffy'.
What causes recycled 'stuffy' air to smell stuffy? Is the sleepiness to do with increased Co2 / decreased O2 due to people breathing? Then is the 'stuffy' smell just the smell of CO2?
Any ideas how big of a change in concentrations we're talking about? Wikipedia says concentration of O2 in air is 20.95%. So are we talking about a drop by less than 1%, or a drop by a few %, etc.?
Is this 'stuffy air' bad for you? e.g. hypothetically if I have trouble sleeping, could I seal my bedroom (as sealed as an ordinary passenger car is), install an air con, and have it start recycling air when it's bedtime. Would this be bad for me long term?