Close the door, shut the windows and turn up the heating, and you have all the ingredients for a sick bedroom in your home – and you spend about eight hours in there every night.

Kitchens and bathrooms usually take the blame for stale air and damp. But researchers spent a winter monitoring airtight, naturally ventilated homes across Scotland. In more than four out of five homes the carbon dioxide (CO₂) in the air remained above the level considered a safe threshold for the entire night. Despite this, more than nine in ten of the occupants thought the air in their bedroom was good.

The reason is not the room. It is you. Awake in the living room, you are a thermostat with hands. When the air turns stuffy, you crack a window. When you feel cold, you close it. This correction stops in bedrooms once you fall asleep, and two things build up: the CO₂ you breathe out and moisture with it.

High levels of CO₂ act as a proxy for stale air. CO₂ levels are measured in parts per million (ppm), a unit of measurement used to describe very small concentrations of a substance.

A study examining 40 Danish bedrooms revealed a troubling threshold: when CO₂ levels reach 1,800ppm, participants’ cognitive function the following day suffered. And sleep quality starts to suffer well........

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