What Changes Overnight CO₂ in a Closed Bedroom?

Key takeaways

• Count the sleepers. Someone joining halfway through the night can change the CO₂ curve even if the window and door stay exactly as they were.
• A door and a window lead to different air. A door connects the bedroom to the rest of the home. A window can bring outdoor air straight in.
• Give the monitor a fixed spot. A shelf clear of pillows, vents and window draughts gives you a fairer comparison across nights.
• Compare the hours everyone was actually in bed. Overlay several nights on the same clock and mark arrivals, departures and changes to the door or window.
• Read the curve as a record of the room. It can help you choose a ventilation setting. On its own, it can’t explain how someone slept.

The question at home

You wake up, open the bedroom window and go looking for your glasses. By the time you check the little monitor on the shelf, it shows a perfectly ordinary number. Then you scroll back. For much of the night the line was up near the top of the screen. Which reading describes the bedroom you slept in?

Both belong to the same room, but the one you see over breakfast belongs to a different part of its day. Scroll back to the hour before anyone got up. Was the line still climbing, had it been level for hours, or did it start falling before you touched the window?

A family detail that seems too small to write down can be the clue. A child came in after a bad dream. Someone on an early shift left and pulled the door shut. Write “one sleeper until midnight, then two” beside the curve and a change of slope suddenly has a possible explanation. If you’re new to reading a trace, start with CO₂ as a ventilation compass.

What physics allows

What goes into the room, and what gets out?

Every sleeper adds carbon dioxide, or CO₂, with each breath. A monitor measures the fraction of the air made up of that gas in parts per million, or ppm. How quickly the number rises depends on how much CO₂ people add, how much air shares it and what air comes in to replace it. A large bedroom gives the same amount of exhaled gas more space to mix. A small room with a good outdoor-air supply can still have a lower overnight reading than a large one with very little flow. (NIST’s room-air model)

Here’s a sense of scale. In two Danish bedrooms, researchers estimated that a sleeping person gave off roughly 8 to 11 litres of CO₂ per hour. Take 10 litres as a round number and a 30-cubic-metre bedroom as an example. That room holds 30,000 litres of air. With perfect mixing and no air exchange, one sleeper would add about 330 ppm in an hour. This deliberately simplified calculation explains how a line can move quite quickly. A real bedroom usually exchanges some air even with the door shut, through gaps or a purpose-built air supply. (Sekhar and colleagues)

Why the line bends

Below are two calculated nights in that same room. Two sleepers enter at 22:00, each giving off 10 litres per hour. Both nights start at 500 ppm, the incoming air is assumed to be at 450 ppm, and the room air mixes evenly. Only the steady outdoor-air flow changes. The numbers are rounded to the nearest 10 ppm. These are examples you can reproduce with a single-room air-balance model.

Look at the last two entries on the right. The sleepers are still breathing. Together they add 20 litres of CO₂ an hour. Once the line levels off, the air going out carries about 20 litres more CO₂ per hour than the air coming in. A level stretch on a real graph can have that explanation too. Opening a window before bed can lower the starting point, yet the curve may climb again once everyone settles in and the window closes.

A bedroom door adds a wrinkle. Open it onto an empty, well-ventilated hall and the bedroom air has somewhere useful to mix. Open it onto a living room where people have spent the evening with the windows shut, and the incoming air may already contain quite a bit of CO₂. (Swiss field measurements, pp. 51–53)

What tests show

The idea that an internal door can change overnight CO₂ even with the bedroom window shut is supported by measurements in one occupied Swiss flat. A bedroom averaged about 1,000 ppm on nights when the door was open nearly the whole time, compared with about 1,600 ppm when it was almost always closed. A more heavily occupied bedroom in the same flat reached a nightly mean of about 3,000 ppm. (Moser and colleagues, figure 52 and accompanying text)

A substantial drop in CO₂ when a bedroom window is opened is supported by a Danish field experiment. The team followed 40 bedrooms over two weeks in winter. In the 16 rooms where the window change produced a clear response, the mean was 1,820 ppm with the window closed and 761 ppm with it open. In 13 other rooms, the corresponding door comparison was 2,362 versus 1,293 ppm. The researchers selected those groups because their readings changed. The two groups can’t tell you which opening will work better at home. (Fan and colleagues, table 3)

The idea that last night’s CO₂ level on its own explains why you woke feeling groggy has been contradicted in a blinded test. Whether stuffy air as a whole affects how you feel the next morning is less clear. In a double-blind trial, 36 children slept under different conditions, including well-ventilated air with pure CO₂ added. The researchers found no significant difference in next-morning cognitive performance across conditions. Wrist monitors actually recorded slightly lower sleep efficiency in the lowest-CO₂ condition, which the authors considered a possible chance finding. The children had also spent time in well-ventilated rooms before the morning tests. At home, opening a window may also let in cool air or the sound of the first bus. A morning note can capture those details better than a CO₂ reading can. (Klausen and colleagues)

What you can do anyway

Set up once, then leave the monitor alone

Use or borrow a monitor that measures CO₂ directly and saves a time history. Look for “non-dispersive infrared” or “NDIR” on the specification. An instrument labelled “eCO₂” may estimate a value from other gases. Check the clock before bedtime. Five-minute logging is a practical choice if the interval is adjustable. (HSE monitor guidance · How one eCO₂ sensor estimates its reading)

Pick a shelf around the height of a sleeper’s head and more than half a metre from the nearest face. Keep it away from an open window, a doorway or a supply vent. Leave the monitor in the same spot throughout the experiment. In one naturally ventilated bedroom researchers could see differences just by changing sensor position near the sleeper. (HSE placement guidance · Bedroom sensor study)

Give each night a label before you look

Start with your usual arrangement. On a comparison night, change one thing you can comfortably repeat. For instance, keep the bedroom door shut both nights and compare your usual ventilation setting with the next normal setting on your home’s system. Or compare your usual closed window with a secure opening suitable for overnight use. Leave permanent vents in service. If the door is normally closed at night for fire safety, keep it closed throughout. (Home fire-safety guidance)

At lights-out, jot down the time, who’s in the room, door and window positions, and the ventilation setting. In the morning, write down anything you remember before opening the graph. “Baby joined us around three” or “woke up cold and shut the window” is enough. Try each arrangement more than once, alternating them if you can. A mild, still night and a windy one may give you different results with the very same window opening. (HSE guidance on repeated measurements)

Put the curves on the same clock

Overlay the nights on the same time axis and use the same vertical scale. Label each line with the sleepers and door position, even if the door stayed shut. Compare an interval when the same people were present throughout, such as midnight to 05:00 if that fits your notes. Look at the average over those hours and at the shape of the line. If bedtime differed, mark when each person arrived so a later start doesn’t get mistaken for better ventilation.

What would make you keep a change? If the curve repeatedly settles lower with a setting everyone finds comfortable, you have a reason to use it. If the clearest difference is that the room felt colder or the traffic woke you, write that down too. And if a closed-door bedroom stays high across comparable nights despite its usual ventilation running, take the graph and your notes to whoever maintains the system. “Two sleepers, door shut, here’s what changed when we turned the supply up” gives them something specific to check.

What if the number and the room disagree?

Is 1,000 ppm a sleep threshold? There’s no sudden change in CO₂ exposure as the display ticks past 1,000 ppm. The familiar round number can prompt a look at ventilation, but no single CO₂ value tells you whether the air is good in every respect or whether someone will sleep well. (ASHRAE’s position on indoor carbon dioxide)

Why is the window wet when CO₂ looks low? Condensation depends on moisture and the temperature of the glass. If wet windows prompted the experiment, add room temperature and relative humidity to your notes. (Mould and moisture without panic)

Will a purifier flatten the curve? An ordinary particle filter can catch airborne particles while the CO₂ line carries on climbing. A lingering paint smell is a different question again. SolarHealth’s air guides cover both particle filtration and gases released by indoor materials.

Sources

• Sekhar C. et al. (2021). Detailed characterization of bedroom ventilation during heating season in a naturally ventilated semi-detached house and a mechanically ventilated apartment.
• Persily A. and Polidoro B. (2022). Indoor Carbon Dioxide Metric Analysis Tool, NIST Technical Note 2213.
• Moser M.-T. et al. (2022). Analyse vereinfachter Lüftungskonzepte, pp. 51–53. Swiss Federal Office of Energy.
• 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.
• Klausen F. B. et al. (2023). The effect of air quality on sleep and cognitive performance in school children aged 10–12 years. Double-blind crossover trial.
• UK Health and Safety Executive. Using CO₂ monitors. Workplace guidance, used here for sensor selection and placement.
• ScioSense. How the ENS160 derives an equivalent CO₂ reading. Sensor manufacturer’s technical note.
• ASHRAE (2025). Position Document on Indoor Carbon Dioxide.
• UK Home Office. Fire safety in the home.