Keeping Doors Closed as One Reason for Fatigue in Teenagers—A Case Study

(1) Background: Healthy teenagers are often sleepy. This can be explained by their physiology and behavioral changes; however, the influence of CO2 concentration above 1000 ppm should not be neglected with respect to sleep dissatisfaction. (2) Methods: CO2 concentrations were measured in two similar bedrooms occupied by girls aged 9 and 13 years old. The scheme of measurements included random opening and closing of the bedroom doors for the night. Additionally, the girls evaluated their sleep satisfaction in a post-sleep questionnaire. (3) Results: During the night, the CO2 concentration varied from 402 to 3320 ppm in the teenager’s bedroom and from 458 to 2176 ppm in the child’s bedroom. When the bedroom doors were open, inadequate indoor air quality (IEQIII and IEQIV categories) was observed in both the teenager’s and child’s bedroom during 11% and 25% of the night, respectively; however, closing the doors increased the contribution of moderate (IEQIII) and low (IEQIV) categories of air to 79% and 86%, respectively. The girls were dissatisfied only when the bedroom door was closed. The satisfied category of sleep was selected only by the younger girl. (4) Conclusions: Opening the bedroom door during the night can decrease the CO2 concentration 55–64% without reducing thermal comfort.