Abstract Ventilation in buildings plays a significant role from the points of view of indoor air quality, health and comfort. In addition, ventilation is one major consumer of heating energy in buildings. In this study, we investigated the effects of shutting down mechanical ventilation at nights on measured and occupant reported indoor air quality as well as health symptoms. Extensive field measurements and standard MM questionnaire surveys among school personnel and children were carried out three months before and three months after changing the ventilation operation mode (from 24 h ventilation to shutting down when unoccupied) in twelve school and daycare buildings. There were small differences in the estimated mean indoor temperature, relative humidity, and particle concentrations results before and after the operation mode change (intervention), which could also be related to seasonal factors. However, the intervention did not associate with any of the studied occupant outcomes, including reported thermal comfort, stuffy ‘bad’ air, or health symptoms among children or personnel. Therefore, according to the survey responses, shutting down ventilation at night had no noticeable effects on the perceived indoor air quality and health during occupancy.