What happens to bedroom air while you sleep

While you sleep, every breath adds a little CO₂ to the bedroom. With the door and window shut, the level usually climbs for the first few hours, then slows and levels off, so the night's high often arrives close to morning.

Where the CO₂ comes from

People breathe out CO₂ all the time, awake or asleep. ASHRAE notes that how much each person produces depends on things like age, body size and activity level. A sleeping adult breathes out less than someone walking around the house, but over a whole night in a closed room it adds up.

The only way that CO₂ leaves is with air that leaves the room. Air can escape under the door, through gaps around windows, or through a vent. If very little air moves in and out, the CO₂ has nowhere to go.

Why the bedroom is the room to check first

During the day, most people move between rooms, and doors open and close. At night, you spend one long, unbroken stretch in the same room, often with the door shut. That makes the bedroom the room where CO₂ has the most time to build up, and the one where a single number tells you the most.

Real homes bear this out. A study of 40 newly built homes in Scotland found that bedrooms averaged 1,520 ppm during occupied nighttime hours, nearly twice the 800 ppm bedroom goal. In bedrooms where the window, door and small window-frame vents were all closed, the average between 11 pm and 7 am was about 2,000 ppm. The bedrooms in that study averaged about 130 sq ft (12 m²). The authors noted that people may go to bed in reasonable conditions that "progressively deteriorate overnight."

What shapes the curve

A few things do most of the work:

• The door. A closed door largely separates the bedroom from the bigger volume of air in the rest of the house.
• The window. A closed window removes the most direct route for outside air.
• Room size. A small room has less air to dilute each breath, so CO₂ climbs faster.
• The number of sleepers. Two people add CO₂ about twice as fast as one. Pets that sleep in the room add some too.

Heating and cooling can play a part as well, so it's worth noting whether yours ran overnight.

What a typical night looks like

Most closed-room nights follow a similar shape. The level starts wherever the evening left it, then rises fastest in the first hour or two after lights-out. As the night goes on, the climb slows.

The slowdown has a simple cause. As CO₂ builds, every bit of air that leaks out of the room carries more CO₂ with it. Eventually the amount carried out each hour roughly matches the amount being breathed out, and the line flattens. Engineers call this steady state. How high that plateau sits depends on how much outside air the room gets for the people in it.

In the morning, opening the door or window usually brings the number down quickly. Small spikes and dips during the night, from someone getting up or the heat coming on, are normal.

A fictional example night

This example is made up to show the typical shape. It isn't from a real home. Picture two adults in a small bedroom with the door and window closed, and the monitor on a dresser across the room.

The first hour adds about 300 ppm. The last hour adds about 30. By early morning the room is close to its plateau, with an overnight high of about 1,600 ppm. That is roughly 1,200 ppm above outdoor air, which sits around 425 ppm.

Averaged over the whole night, this room comes to about 1,320 ppm. That average is the number to compare with the goal of 800 ppm or less while you sleep, because it's what the sleep studies measure. This room is more than 500 ppm over, and well into the range where controlled studies see people sleeping worse.

Notice that the 10 pm reading looked fine. Someone checking only at bedtime would have missed the whole story.

What this means for measuring

A bedtime glance isn't enough. To understand a bedroom, you need the whole night. A monitor that records (or the HouseExhale app listening overnight) gives you the night's average. Without one, the highest reading, or the one first thing in the morning before anyone opens the door, is a fair upper estimate: the average is always a little lower.

One night can also be unusual. A guest, a pet, a window left open: any of these can change the curve. The UK Health and Safety Executive advises recording CO₂ along with the number of people and the type of ventilation in use, and that is the habit to build. HouseExhale asks for three nights for the same reason, so a pattern can stand out from any one odd night.