High CO2 levels in bedrooms at night are linked to worse sleep and functioning the next day, and increase the chance of being exposed to things like mould, bacteria and dust mites. Researchers found that half of the 20 bedrooms they surveyed in Wellington over winter exceeded the commonly recommended CO2 levels for much of the night, with some reaching very high concentrations. The researchers say this could be because most of New Zealand's housing stock is 'naturally ventilated' through windows and doors, which are commonly kept closed at night.