A reasonably popular brand's product that uses an NDIR sensor revealed to me just how much the CO2 level increases each night in my two bedrooms.

One of them seems to have much worse ventilation to the extent that it reaches double the level. Opening the window slightly 24/7 keeps it low.

My fiance's chronic headaches/migraines/idk became noticeably less frequent after this change and when they do occur it's usually because the window was accidentally left closed.

Anybody who struggles with this kind of thing might want to try checking their levels. Or just open a window I guess?