Why does my bedroom feel stuffy at night?

Two small studies in Danish student rooms compared a week of fresher air with a week of stuffier air. With fresher air, the room felt fresher, and students said they felt less sleepy and could concentrate better the next day².

A study of 62 adults tracked their own bedrooms for 14 nights. On the stuffiest nights, they spent about 4% less of their time in bed sleeping than on the freshest nights³. That shows two things happening together. It doesn't prove one caused the other.

A 2025 review of 17 studies suggests keeping bedroom carbon dioxide below 1,000 parts per million, and preferably below 800¹. Those researchers treat the figure as a sign of how well a room is ventilated, not as a pollutant¹.

• A window open a little. This is what the first Danish study changed². Not tonight? That's fine.
• A window that can't stay open. Burglar bars, cold, noise and safety are all good reasons. An open bedroom door is worth a try instead, though the evidence is mixed. One study of 84 Danish bedrooms found people rated their sleep better with a door or window open⁴. A later study that tested it directly saw the gains with the window, and not with the door⁵.

This one matters more than comfort. Eskom's winter safety advice is to keep at least one window slightly open, and never to leave heaters on unattended, especially overnight or while sleeping⁶.

A carbon dioxide reading won't warn you about carbon monoxide, which is a different gas. Dormio doesn't check for carbon monoxide and isn't a safety alarm.

Dormio sits by the bed and keeps an eye on the room: fresh air, warmth, humidity, light and sound. It looks at the room, not at you.