Poor bedroom ventilation, leading to poor indoor air quality (IAQ), has been shown to reduce sleep quality. Ventilation causing a carbon dioxide (CO2) concentration of 1,000 ppm is assumed to permit optimal sleep quality. The effects of three ventilation levels causing average indoor CO2 concentrations of 750 ppm, 1,000 ppm, and 1,300 ppm, on sleep quality, physiological response, and next-day work performance were examined. After a first night for adaptation, thirty-six young and healthy participants slept alone in simulated bedrooms for two nights at each of the three ventilation levels in balanced order. Sleep quality was recorded by a wrist-worn sleep tracker. Physiological parameters were measured before sleep and after waking.⁠ ⁠ The participants’ ratings of the bedroom environment, the intensity of the health symptoms they experienced, and cognitive performance were obtained using questionnaires and tests.⁠ ⁠ Compared with ventilation causing an average CO2 concentration of 750 ...