Your Bedroom's CO2 Level Overnight Is Probably Higher Than You Think

If you sleep with the door shut and the window closed, the air around you by morning is measurably different from the air you fell asleep in. That's not just a stuffy feeling. It's carbon dioxide building up all night, and recent research suggests it's doing more to your sleep than a stuffy feeling would imply.

Here's what that buildup actually looks like in numbers, and what the research says it costs you.

The math for a real bedroom

Take a small shared bedroom, 3.5 by 3 by 2.4 meters, about 25 cubic meters of air, with two people sleeping in it. Sleeping bodies produce less CO2 than people up and moving around, roughly 40% less, so the numbers below use that lower, sleeping-specific rate.

Sealed up for the night, door and window both shut, that room settles at around 3,370 ppm of CO2 by the time it reaches steady state. Crack the door for a bit of airflow and it drops to roughly 1,300 ppm. Crack a window instead, which moves more air than a cracked door, and it falls to about 715 ppm.

You can run these numbers for your own room, your own headcount, and your own ventilation on our Indoor CO2 Ventilation Calculator. Set the activity level to Sleeping and plug in your bedroom's actual dimensions.

What those levels mean for your sleep

A 2016 study in the journal Indoor Air put real subjects in dorm rooms and varied only the ventilation, one week at each setting.

In one experiment, opening a window dropped the average bedroom CO2 from 2,585 ppm to 660 ppm. In a second experiment, a fan that kicked in above 900 ppm brought the average down from 2,395 ppm to 835 ppm. Either way, the lower-CO2 week came with measurably better sleep: actigraph-tracked sleep quality improved, and subjects reported less next-day sleepiness, better concentration, and scored better on a test of logical thinking.

Why 800 ppm keeps coming up

That 2016 study didn't pin down an exact safe threshold, just that lower was better within the range it tested. A newer research project aimed at exactly that question, sponsored by ASHRAE and run out of the Technical University of Denmark, tried to find the line.

Its conclusion, reported by EurekAlert: 850 ppm was the highest level that caused no measurable effect on sleep in their data, but they didn't trust that number as a safety margin because of how CO2 sensors drift, so they proposed keeping bedrooms at or below 800 ppm instead. Hitting that target, they found, takes roughly 8 liters of outdoor air per second per person, about twice what many current residential ventilation codes actually require.

Run backward through the same room from above, hitting 800 ppm for two sleeping adults in that 25 cubic meter bedroom takes about 2.3 air changes per hour, more than a cracked door gets you, a bit less than a cracked window.

What actually helps

The gap between "sealed" and "door cracked" is the single biggest lever here, a jump from roughly 3,370 ppm down to 1,300 ppm from one small change. A cracked window gets a typical bedroom close to the 800 ppm range the newer research points to.

If cracking anything isn't an option, for noise, temperature, or security reasons, an exhaust fan or a trickle vent sized for your actual room does the same job without leaving anything open. Either way, the fastest way to know where your own bedroom lands is to measure it: plug your room into the calculator above with a couple of real ventilation settings and see which one gets you under 800 ppm.