Impact of carbon dioxide exposures on sleep latency among healthy volunteers: A randomized order, paired crossover study, evidence from the multiple sleep latency test.

Background: Daytime sleepiness affects work efficiency, occupational safety, and public health. Although previous studies have reported an association between environmental carbon dioxide (eCO ₂ ) and daytime sleepiness, it has been challenging to draw a firm conclusion due to the lack of standardized sampling and profiling protocols.
Objective: We examined the effect of pure CO ₂ exposure at 5000 (ppm, parts per million) on daytime sleepiness.
Methods: Eleven healthy participants (males of 24 ± 3 years, mean ± SD) completed a four-nap multiple sleep latency test (MSLT) protocol in the environmentally controlled chamber under two conditions: the CO ₂ condition (4851 ± 229 ppm) and the Control condition (1102 ± 204 ppm). The subjective sleepiness level and cognitive performances were also evaluated using the Stanford Sleepiness Scale (SSS) questionnaire, Psychomotor Vigilance Test (PVT), and Stroop test after each nap session.
Results: A significant reduction in sleep latency was observed in the CO ₂ exposure condition (Control vs. CO ₂  = 13.1 ± 3.3 min vs. 9.7 ± 3.2 min). The subjective sleepiness scores were also significantly higher in the CO ₂ exposure condition than in the Control condition (Control vs. CO ₂  = 2.7 ± 0.5 vs. 4.7 ± 0.8). Cognitive responses after naps showed no significant difference across conditions.
Conclusion: This study revealed that exposure to environmental CO ₂ at a concentration as high as the upper safety limit at work sites significantly shortened the sleep latency and enhanced subjective sleepiness during naps in the MSLT without affecting cognitive responses after each exposure. Our results demonstrated that exposure to high environmental CO ₂ induces daytime sleepiness that potentially compromises work efficiency and safety.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)