Objectives: To investigate the effect of CO₂ during sleep on next-morning cognitive performance in young schoolchildren, the authors performed a double-blind fully balanced crossover placebo-controlled study. Material and Methods: The authors included 36 children aged 10–12 years in the climate chamber. The children slept at 21°C in 6 groups each at 3 different conditions separated by 7 days in a random order. Conditions were as follows: high ventilation with CO₂ at 700 ppm, high ventilation with added pure CO₂ at 2000–3000 ppm, and reduced ventilation with CO₂ at 2–3000 ppm and bioeffluents. Children were subjected to a digital cognitive test battery (CANTAB) in the evening prior to sleep and on the next morning after breakfast. Sleep quality was monitored with wrist actigraphs. Results: There were no significant exposure effects on cognitive performance. Sleep efficiency was significantly lower at high ventilation with CO₂ at 700 ppm which is considered to be a chance effect. No other effects were seen, and no relation between air quality during sleep and next-morning cognitive performance was observed in the children emitting an estimated 10 l_CO2/h per child. Conclusions: No effect of CO₂ during sleep was found on next day cognition. The children were awakened in the morning, and spent from 45–70 min in well-ventilated rooms before they were tested. Hence, it cannot be precluded that the children have benefitted from the good indoor air quality conditions before and during the testing period. The slightly better sleep efficiency during high CO₂ concentrations might be a chance finding. Hence, replication is needed in actual bedrooms controlling for other external factors before any generalizations can be made. Int J Occup Med Environ Health. 2023;36(2):177–91