What happens to bedroom CO₂ overnight
A single adult breathing in a small closed bedroom adds roughly 250–400 ppm of CO₂ per hour. Starting at outdoor baseline (~420 ppm) at midnight, a 12 m² room with the door and window shut routinely passes 2,000 ppm by 6 AM — and that figure doubles in a shared bed.
This CO₂ is not the toxic part of breath — but it is a perfect proxy for how much of the air around your face has already been exhaled. By morning, half the air you are inhaling came from your own (or your partner's) lungs.
This CO₂ is not the toxic part of breath — but it is a perfect proxy for how much of the air around your face has already been exhaled. By morning, half the air you are inhaling came from your own (or your partner's) lungs.
What the research shows
Controlled studies (Strøm-Tejsen et al, 2016; Mishra et al, 2019) have shown measurable effects of overnight bedroom CO₂ on:
- Sleep efficiency — the proportion of time in bed actually spent asleep
- Number of awakenings per night
- Self-reported drowsiness on waking
- Next-morning cognitive performance on standardised tests
The largest effects appear around 2,000–2,500 ppm; differences below 1,500 ppm are smaller but still measurable. The simplest takeaway: keep bedroom CO₂ below 1,500 ppm overnight where you can.
- Sleep efficiency — the proportion of time in bed actually spent asleep
- Number of awakenings per night
- Self-reported drowsiness on waking
- Next-morning cognitive performance on standardised tests
The largest effects appear around 2,000–2,500 ppm; differences below 1,500 ppm are smaller but still measurable. The simplest takeaway: keep bedroom CO₂ below 1,500 ppm overnight where you can.
How to lower overnight bedroom CO₂
Three interventions, ranked by effectiveness:
1. Open the bedroom door. The single biggest free win. Even slightly ajar typically halves overnight CO₂ in a small room.
2. Crack a window. Just a few centimetres of bottom-sash opening provides enough trickle-vent flow to keep most rooms below 1,500 ppm overnight, even with the door closed.
3. Install a trickle vent or run mechanical ventilation. For homes with security or noise concerns that rule out an open window. PIV (positive input ventilation) systems work well for the whole house.
Watch the next morning's CO₂ curve to see whether your intervention worked. It should drop the overnight peak by 500+ ppm.
1. Open the bedroom door. The single biggest free win. Even slightly ajar typically halves overnight CO₂ in a small room.
2. Crack a window. Just a few centimetres of bottom-sash opening provides enough trickle-vent flow to keep most rooms below 1,500 ppm overnight, even with the door closed.
3. Install a trickle vent or run mechanical ventilation. For homes with security or noise concerns that rule out an open window. PIV (positive input ventilation) systems work well for the whole house.
Watch the next morning's CO₂ curve to see whether your intervention worked. It should drop the overnight peak by 500+ ppm.
How Envora One helps
Envora One sits on a bedside table or bookshelf, reading CO₂ (true NDIR), temperature, humidity and sound through the night. The dashboard surfaces the overnight CO₂ peak and average — so you can see, within a single night, whether opening the door, cracking a window or running the MVHR materially changed the room's air.
For parents: it answers 'does my child's bedroom need a window cracked?' with data instead of guesswork. Buy Envora One — £249.
For parents: it answers 'does my child's bedroom need a window cracked?' with data instead of guesswork. Buy Envora One — £249.