ELI5? - Your normal city air CO2 is 350-500 PPM. - Humans exhale CO2. - In closed unventilated bedroom CO2 have nowhere to go. - Air isn't being mixed when

ELI5? - Your normal city air CO2 is 350-500 PPM. - Humans exhale CO2. - In closed unventilated bedroom CO2 have nowhere to go. - Air isn't being mixed when you sleep too. - In few hours PPM can grow to 2000 or higher. - Cheap air conditioners either dont have fresh air ventilation at all or dont have sufficient efficiency to compensate for two humans breathing. You can google actual numbers elsewhere. Solution usually consist to sleeping with open windows or getting proper fresh air ventination system.