Measurements of air change rates, carbon dioxide concentrations, room air temperatures and relative room air humidities in bedrooms of five well tightened dwellings were carried out in October 1989. With the results of the measurements and also based on simulation calculations, recommendations for an optimal window opening behaviour in bedrooms with the intention of saving energy and of providing sufficient indoor air quality were made. Air change rates between the outdoor air and the bedroom air, and between the bedroom air and the rest of the dwelling measured with the constant concentration method varied between 0.01 and 12.3 h^-1, corresponding to an air supply of 0.16 to 320 m³ h^-1 pers^-1 depending on inhabitant behaviour. The measured carbon dioxide concentrations at the end of the night varied between 515 and 4286 ppm. The average room air temperatures ranged from 20.1 °C to 22.3 °C and the relative humidities from 42.4 % to 64.7 %.