Bedroom ventilation is crucial for providing good indoor air quality, which may contribute to achieving undisturbed sleep. This study presents a detailed characterization of bedroom carbon dioxide (CO2) profiles and ventilation during the heating season in Denmark. The measurements were made in a naturally ventilated (NV) semi-detached house, equipped with trickle vents on the windows and doors (augmented if needed with exhaust fans), and a mechanically ventilated (MV) apartment. The location of sensors in and around the immediate breathing zone of a sleeping person had measurable effect on the absolute level of CO2 only in the NV bedroom and not in the MV bedroom. Using the decay of occupant-generated CO2, the estimated breathing zone air change rate (ACR) was around 0.4 h−1 and 0.6 h−1 for the NV and MV bedrooms respectively. This was adequate for an average breathing zone CO2 level of 1000 ppm for the MV bedroom but not for the NV bedroom. The CO2 emission rate of 7.9–10.9 L/h.person during sleep, obtained from bedroom CO2 measurements, was lower than the estimated values of 12.8–13.9 L/h.person, using currently proposed methods in standards. Thermal comfort parameters during all the experimental sessions in both bedrooms were well within recommended levels.