Effects of Dynamic Increases in Indoor CO2 Concentration on Daytime Sleepiness: An EEG-Based Study

Elevated indoor carbon dioxide (CO2) concentrations are increasingly recognized as a critical determinant of occupant health and cognitive performance; however, the neurophysiological mechanisms linking CO2 exposure to daytime sleepiness remain a subject of debate. This study investigated the effects of dynamically rising CO2 concentrations on subjective sleepiness and neural oscillation patterns in a controlled environment. A within-subject repeated-measures experiment was conducted with 18 healthy university students exposed to CO2 levels gradually increasing from a baseline of ~800 ppm to ~2000 ppm over a 40-min period. Subjective sleepiness was assessed using the Karolinska Sleepiness Scale (KSS), while electroencephalogram (EEG), heart rate (HR), and heart rate variability (HRV) were monitored continuously. Results indicate a significant increase in subjective sleepiness scores concurrent with rising CO2 levels (p < 0.001). Power spectral density (PSD) analysis revealed that elevated CO2 exposure significantly enhanced power in low-frequency EEG bands (δ, θ, and α), indicative of a transition from alertness to drowsiness. Specifically, α-band power peaked between 20 and 30 min (increasing by 0.15 uV2 relative to the 0–10 min baseline), while θ and δ bands peaked earlier (10–20 min) and sustained elevated levels through 30 min. Topographic mapping identified the central, parietal, and occipital regions as the primary loci of this low-frequency activity. Additionally, HR and HRV measures showed upward trends, suggesting autonomic modulation. It is noted that low-frequency EEG power declined during the final 30–40 min interval, potentially reflecting physiological saturation or acclimatization. These findings provide objective neurophysiological evidence that dynamic CO2 accumulation accelerates drowsiness onset, underscoring the necessity of optimized ventilation strategies in educational settings. However, these findings should be interpreted with caution due to the small sample size and the absence of a control group with constant CO2 concentration. Future studies with larger, diverse samples and stable-concentration control groups are recommended to validate these trends.