CO₂ in a closed bedroom overnight, and what it does to sleep

A closed bedroom with two sleepers can pass 1,500 ppm of CO2 within hours; small studies link such rooms to lighter sleep, and CO2 is best read as a gauge of ventilation.

Most Indian bedrooms with a split air conditioner are run the same way at night: windows shut, door shut, AC on. The room stays cool because the same air is cooled again and again. Nothing in that arrangement brings in outdoor air, so whatever the sleepers breathe out stays in the room until morning.

Carbon dioxide (CO₂) is the easiest part of that to measure. This article works out how fast it builds up and sets out what sleep researchers have found, including what is still uncertain.

The arithmetic of a closed room

A resting adult breathes out roughly 0.003 to 0.004 litres of CO₂ every second, or about 12 litres an hour ^[1]. Two sleepers produce about 24 litres an hour.

Take a bedroom of 12 square metres with a 2.8 metre ceiling: 34 cubic metres, or 34,000 litres of air. Outdoor air now carries about 425 ppm of CO₂ ^[2]. If the room were perfectly sealed, two sleepers would add about 700 ppm every hour.

No room is perfectly sealed. What matters is how many times an hour the room's air is replaced. For this room and two sleepers, the same sum gives:

These are calculations, not measurements, and they assume evenly mixed air. A tighter room ends the night at a much higher level. The level a room settles at depends on the fresh air per person, not on room size; a bigger room only takes longer to get there.

What measured bedrooms show

Field studies report numbers in the same range. In student dormitory rooms with one occupant each, the average overnight level was 2,585 ppm with the window closed and 660 ppm with it open; in a second experiment it was 2,395 ppm with the room's ventilation fan off and 835 ppm with it on ^[3]. In a study of 17 healthy adults, bedrooms averaged 1,150 ppm with window and door closed and 717 ppm with one of them open ^[4].

We did not find a published study that measured CO₂ overnight in Indian bedrooms. Readings from local homes would be more useful here than figures from other countries.

What the sleep studies found

The evidence comes from a small number of small studies. Four are worth knowing.

• Strøm-Tejsen and colleagues, 2016. Students slept for a week in each condition. When CO₂ was lower, sleep quality measured by wrist movement sensors improved, the students reported less sleepiness and better concentration the next day, and they did better on a test of logical thinking ^[3].
• Mishra and colleagues, 2018. With a window or door open, people reported deeper sleep, and the movement data also differed between the two conditions ^[4].
• Fan and colleagues, 2023. Researchers changed the fan speed of the bedroom ventilation in 29 homes without telling the occupants. At lower ventilation rates people had less deep sleep, more light sleep and more awakenings. Tests of mental performance, taken in the evening and morning, showed no difference ^[5].
• Basner and colleagues, 2023. Sixty-two adults had their bedrooms monitored for two weeks. Sleep efficiency was 4.0% lower in the fifth of readings with the highest CO₂ than in the lowest fifth. Heat, noise and fine particles each had an effect of similar size ^[6].

A 2025 review brought 17 such studies together. It found disturbed sleep where bedroom CO₂ reached about 1,000 ppm, found no apparent effect at about 850 ppm, and proposed keeping bedrooms at or below 800 ppm. It estimated that this takes about 8 litres of outdoor air per second for each person, at least twice what many home ventilation standards ask for ^[7].

An earlier review was more guarded: air quality appears to influence sleep, but "the evidence is limited" ^[8].

Is it the CO₂ itself?

Probably not, at these levels. CO₂ rises together with everything else that people and furnishings give off in a closed room, and with humidity. In a laboratory study that separated the two, adding pure CO₂ up to 3,000 ppm to well-ventilated air had no significant effect on how people felt or performed. Reaching the same CO₂ level by cutting ventilation did: people reported more headache, fatigue and sleepiness ^[9].

ASHRAE, the engineering body that writes ventilation standards, takes the same view. Its position is that the evidence for direct effects of CO₂ on health and performance at ordinary indoor levels is inconsistent, and that CO₂ is a useful tool in assessing indoor air only if its limits are understood ^[10]. The 2025 review calls CO₂ a proxy for ventilation rate, not a pollutant ^[7].

For a household the practical point is simple. A high CO₂ reading says the room is not getting enough outdoor air, and ventilating lowers everything else that had built up with it.

What about the next day?

Results are mixed. The 2016 study measured better next-day concentration and logical thinking ^[3]. The 2023 field study, which was larger and kept its participants unaware of the changes, measured better sleep but no change in performance ^[5]. Deeper sleep has been found in several studies; a next-day gain in some and not in others.

What ventilation does

Go back to the 34 cubic metre bedroom. At 8 litres per second for each sleeper, two people need about 58 cubic metres of outdoor air an hour. With that supply the same arithmetic puts the room at about 840 ppm all night, close to the level the review proposes, in place of 1,500 to 3,000 ppm.

An open window can deliver that much air. In an Indian city it also lets in heat, humidity, noise and, in winter in the north, heavily polluted air. Heat and noise were as strongly tied to poor sleep as CO₂ in the 2023 monitoring study ^[6], so trading one for the other gains little.

Mechanical ventilation is the alternative: a unit that supplies filtered outdoor air and removes the same amount of room air, continuously and with the windows shut. An energy recovery ventilator passes the two air streams through an exchanger so that the outgoing cool air pre-cools the incoming air. InAir's residential units start at 150 cubic metres an hour and can be linked to air-quality sensors so that airflow follows the room's needs.

Checking your own bedroom

A CO₂ monitor will show the pattern within a night or two. ASHRAE notes that a sensor's accuracy, position and calibration all affect what the reading means ^[10]. Keep it away from the pillow and read it on waking, before the door is opened.

This article is general information. It is not medical advice, and no ventilation product treats a sleep disorder.

Sources

1. Persily A, de Jonge L. "Carbon dioxide generation rates for building occupants." Indoor Air 2017;27(5):868–879. https://pmc.ncbi.nlm.nih.gov/articles/PMC5666301/ (Table 4: 0.0029 to 0.0037 L/s for adults aged 30 to 40 at rest.)
2. NOAA Global Monitoring Laboratory, global annual mean CO₂ (425.62 ppm for 2025). https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_gl.txt
3. Strøm-Tejsen P, Zukowska D, Wargocki P, Wyon DP. "The effects of bedroom air quality on sleep and next-day performance." Indoor Air 2016;26(5):679–686. https://doi.org/10.1111/ina.12254
4. Mishra AK, van Ruitenbeek AM, Loomans MGLC, Kort HSM. "Window/door opening-mediated bedroom ventilation and its impact on sleep quality of healthy, young adults." Indoor Air 2018;28(2):339–351. https://doi.org/10.1111/ina.12435
5. Fan X et al. "A single-blind field intervention study of whether increased bedroom ventilation improves sleep quality." Science of the Total Environment 2023;884:163805. https://doi.org/10.1016/j.scitotenv.2023.163805
6. Basner M et al. "Associations of bedroom PM2.5, CO2, temperature, humidity, and noise with sleep: An observational actigraphy study." Sleep Health 2023;9(3):253–263. https://pmc.ncbi.nlm.nih.gov/articles/PMC10293115/
7. Akimoto M, Fan X, Lan L, Sekhar C, Tanabe S, Wyon DP, Wargocki P. Science and Technology for the Built Environment 2025;31(8). https://doi.org/10.1080/23744731.2025.2531317 Publisher's summary: https://www.eurekalert.org/news-releases/1102252
8. Sekhar C et al. "Bedroom ventilation: Review of existing evidence and current standards." Building and Environment 2020;184:107229. https://doi.org/10.1016/j.buildenv.2020.107229
9. Zhang X, Wargocki P, Lian Z, Thyregod C. "Effects of exposure to carbon dioxide and bioeffluents on perceived air quality, self-assessed acute health symptoms, and cognitive performance." Indoor Air 2017;27(1):47–64. https://doi.org/10.1111/ina.12284
10. Persily A et al. "ASHRAE's New Position Document on Indoor Carbon Dioxide." ASHRAE Journal 2022;64(5):50–52. https://www.nist.gov/publications/ashraes-new-position-document-indoor-carbon-dioxide (the authors' summary of the Position Document.)

Note on the worked figures: the tables and the 840 ppm figure come from a single-zone mass balance, C(t) = C_outdoor + (G / (n × V)) × (1 − e^(−n × t)), with G = 23.8 L/h of CO₂ for two sleepers, V = 34 m³, n in air changes per hour and outdoor CO₂ of 425 ppm.