A closed bedroom averaged six times the CO2 of outdoor air
Fresher air meant falling asleep faster and waking more rested. The fix takes one second.
With the window shut, the air in a one-person bedroom averaged 2,585 parts per million of carbon dioxide through the night. Outdoor air sits around 420.
That is one sleeper, breathing out all night into a sealed room. Add a partner and the number climbs faster.
Carbon dioxide at that level is not poisonous. It is a marker of stale air, and stale air is what this study changed.
Your brainstem watches CO2 closely, because it uses that reading to set how deeply you breathe. The likely explanation is that stale, CO2-heavy air keeps that system working harder and leaves sleep a little lighter, though nobody has pinned down the pathway.
The morning is where it shows. With fresher air, people felt more rested the next day, and next-day sleepiness leaned the same way.
The study, published in Indoor Air in 2016 by researchers at the Technical University of Denmark, ran two experiments in student dorm rooms with equal numbers of men and women. In the first, they compared nights with the window open against nights with it shut.
Opening the window dropped CO2 to about 660 and shortened the time it took to fall asleep. In the second, a silent fan the students did not know about brought CO2 down to about 835, and sleep efficiency on a wrist monitor went up.
Now the honest part. The samples were small, 14 and 16 students, and many single results were only borderline until the two experiments were combined.
An open window also brought drier lips for some, and the fan brought drier mouths and skin. Fresh air comes with trade-offs.
If outside air is loud, cold, or polluted, open the bedroom door to the rest of the home instead. The study tested windows and fans, not doors, but the principle is the same: give the air somewhere to go.
A sealed bedroom hit 2,585. An open window brought it to 660, and the sleep followed the air.
If you have asthma or allergies and pollen or pollution is high outside, keep the window shut that night and use the door instead.
