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By Melissa Fleur Afshar
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The room where most people spend roughly a third of their lives may also be the room doing them the most harm, according to sleep and air-quality experts.
Back in 2015, researchers monitored airtight, naturally ventilated homes across a winter in Scotland. In more than four out of five homes, CO2 levels stayed above safe thresholds all night. Yet more than nine in 10 occupants believed their bedroom air was fine. Experts reviewing the findings in 2026 say the same trapped-air problem is now resurfacing as a summer risk, as heatwaves strain homes not designed for them, while global temperatures continue to rise.
"Global warming causes drastic and abrupt shifts in temperatures," Devjoy Dev, a biomedical engineer who studied the neuromodulation of sleep circuitry for his Ph.D. at New York University, told Newsweek. "This has led to sudden, extreme heat waves across Europe where AC infrastructure isn't readily in place."
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Dev, who also trained at Harvard Medical School, said the effect is worsened by bedroom fabrics such as curtains and carpets, which trap heat and moisture overnight.
Dr. Ayesha Bryant, clinical adviser at Alpas Wellness in Maryland, told Newsweek that the study found average nighttime carbon dioxide levels in bedrooms "exceeded roughly 1,520 parts per million (ppm)." High levels of CO2 act as a proxy for stale air, while its levels are measured in parts per million (ppm).
"A room can have a very comfortable temperature while air from exhaling humans and moisture build-up continuously occur," Bryant said, noting that trapped CO2 and heat can degrade sleep long before anyone notices a problem.
But with high nighttime temperatures in airtight rooms still very much a thing, how does this impact our health over time?
A Room With No Thermostat
Experts say the problem is not actually the room, but what stops happening once its occupant falls asleep. While awake, people instinctively open a window when air feels stale or close it when they are feeling cold. That self-correction ends overnight, allowing CO2 and moisture to build up.
Bryant noted that CO2 itself is not dangerous at these levels, but it signals poor air exchange linked to worse sleep and next-day performance.
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A 2022 study of 40 Danish bedrooms found cognitive function suffered once CO2 reached 1,800 ppm, with sleep quality declining well before that point, which can lead to several health issues over time.
"It's important to strongly keep in mind the sleep environment and its 'zeitgebers,'" Dev said, referring to the environmental cues—light, eating schedules, temperature—that regulate the body's circadian rhythm.
"Controlling these in your bedroom is vital for achieving quality sleep because they can degrade when bedroom CO2 and moisture levels rise," he added.
Clearing the Air
Poor supply and return airflow, closed doors, weak insulation and west-facing windows all trap heat in bedrooms.
Amerisleep research from June provided to Newsweek found 52 percent of Americans identify as hot sleepers, and 94 percent of them wake up weekly because of it, compared with 58 percent of cold sleepers.
Women were twice as likely as men to wake three or more times a week from heat. The most common ideal bedroom temperature is 65–67°F, yet the most common actual temperature is 68–70°F.
Bryant recommends keeping trickle vents unobstructed and airing rooms when outdoor temperatures allow.
The German practice of "Lüften," now popularized among Gen Z on TikTok and Instagram as "house burping," involves opening windows fully for 10 to 15 minutes each morning to purge stale, humid air.
Dehumidifiers, retailing on average for £90 to £300, can hold humidity near the recommended 60 percent.
For persistently hot rooms, fixing heating, ventilation, and air conditioning (HVAC) systems or adding dedicated air conditioning is advised—and any mold or condensation should be treated at the source, not just wiped away.
Reference
Fan, X., et al. (2022). A field intervention study of the effects of window and door opening on bedroom IAQ, sleep quality, and next-day cognitive performance. Building and Environment. https://doi.org/10.1016/j.buildenv.2022.109630.
Sharpe, T., et al. (2015). Occupant Interactions and Effectiveness of Natural Ventilation Strategies in Contemporary New Housing in Scotland, UK. International Journal of Environmental Research and Public Health. https://doi.org/10.3390/ijerph120708480.
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