A few years ago I built a small CO2, temperature and RH logger (some here have borrowed it) and was a bit surprised at how high the bedroom CO2 level got in the middle of the night at our old house, even with a small window that's always left open.  Interesting to correlate events like the bedroom door opening with a sudden decrease in CO2 concentration; demonstrates well how important cross flow ventilation is in a room (our bedroom door in the old house was almost diagonally opposite the small