Based on some HN comment from a while ago I invested in a CO2 meter (they are still quite expensive for some reason). And I share the same experience, CO2 levels can raise rapidly indoors, but simply turning on ventilation or opening a window very quickly lowers CO2 contents.
Using the meter I found CO2 levels in my bedroom can become quite high at night. So I improved the ventilation in my bedroom, and in my case it helped me to achieve better sleep.