@mike_bowler Since getting a CO2 monitor, I've been much more conscious of the effect of stale air when driving on long trips. Now I know that when I start to feel sleepy, it's invariably a signal to open the windows and let in fresh air.
In hindsight, its kind of astonishing that cars don't include a CO2 monitor for ventilation. Just as you can set the desired temperature in the car, it should also be able to regulate ventilation. I wonder how many car crashes it would prevent.