The air goes stale with the door shut, and a CO2 reading will show how fast
When the door and window stay shut, the carbon dioxide (CO2) a sleeper breathes out builds up all night, and an overnight reading shows how fast it rises past the level linked to poorer sleep.
In short: Read CO2 overnight with an NDIR monitor. If it climbs past 1,000 ppm, open the window or the door a little and read it again.
What to check
| What to check | The specification |
|---|---|
| What to measure | Measure the CO2 in the bedroom air across the whole night, with the door and window left as they usually are. A review of bedroom research treats CO2 as an indicator of ventilation sufficiency, not a pollutant (ASHRAE 1837-RP review). |
| The instrument | Use a monitor with a non-dispersive infrared (NDIR) sensor that keeps a log. The HSE recommends that the most appropriate portable devices to use in the workplace are non-dispersive infrared (NDIR) CO2 monitors (HSE advice, as reported by Twinfm). Sensors that estimate CO2 instead of measuring it are a poorer fit, because metal-oxide sensors are not selective to CO2 and their estimates are affected by volatile organic compounds such as alcohol and formaldehyde (US patent 12487216). |
| The range that matters | The review's figures for sleep say CO2 should, as a minimum, remain below 1,000 ppm, and preferably below 800 ppm (ASHRAE 1837-RP review). A field study sums up earlier work this way: levels above 1100 ppm have consistently been shown to have negative effects on sleep quality (field intervention study). For homes, Health Canada sets a long-term (24 h) limit of 1,000 ppm (Health Canada). |
| Where to put it | Workplace guidance says to place CO2 monitors at head height and away from windows, doors, or air supply openings, and at least 50cm away from people (NI Business Info). In a bedroom, that means a shelf or chest near pillow height, not the windowsill and not right beside the sleeper's face. |
| What to change first | Open the window a little. In one dormitory study, opening a window gave an average CO2 level of 660 ppm, against 2585 ppm with it closed (dormitory study). Across twenty-nine bedrooms, sleep was longer and rated better with windows open, but similar effects were not seen with doors open, even though CO2 fell by as much (field intervention study). |
Why your answers lead here
Two answers carry this result. You wake heavy-headed in a stuffy room, and the door and window stay shut, so the same air gets breathed again and again until morning. In a study of 17 healthy sleepers, average CO2 was 717 ppm with a window or door open and 1150 ppm with both closed (Mishra et al., Indoor Air). The dormitory researchers found that when the window is shut to save energy and the door is shut for privacy, CO2 levels routinely exceed 2500 ppm. A logged reading turns the stuffy feeling into a number to check against the range in the table.
How fast CO2 climbs depends on the evening as much as on the night. In the dormitory study, the main source of the variation was the time for which the room had been occupied prior to bedtime. An evening spent in the room with the door closed means the night starts from a higher figure.
What changes it
Your other answers change what to fix first, not what to measure. On a top floor, or when summer is worst, an open window also lets out the heat the building holds, and the guide to bedroom temperature for sleep gives the figure to aim for. When winter is worst and warm air comes through vents, a window open all night costs heat and dries the room further. Ventilating a bedroom in winter covers how to air the room without losing that warmth. A ground-floor room runs damp, so read humidity alongside CO2 and compare it with the humidity range for a bedroom. With nothing new in the room, a smell is more likely to come from damp than from new materials.
A window onto a busy street can help and hurt at once. In a summer study in a dense subtropical city, CO2 concentrations were lower and the air temperatures and PM2.5 concentrations were higher when windows were open (summer window study). On a loud road, setting up white noise may let the window stay open.
What to check on your own bed
Log a few ordinary nights with the room as you keep it, then a few with the window open a crack. Then compare the two. Look at the overnight average and the peak before dawn, because one reading at bedtime says little about the small hours. Keep the monitor at least 50 cm from the sleeper's face, since a sensor beside the pillow reads breath rather than room air. If closed nights sit above 1,000 ppm and open nights come in clearly lower, the heavy head has a likely cause and a fix to try. If closed nights stay low too, the air is changing enough, and heat or humidity is the next thing to read.