[pitspotter]

The idea isn't to go to sleep but to avert a full-blown cold or other respiratory infection if possible. Usually takes 15-45 minutes. But you have to catch it right at the start and you have to be at home, obviously.

If you're concerned about carbon dioxide you can either refresh the air periodically or you can create a small gap in the duvet to allow some exchange of air with the surrounding room (whilst keeping the humidity relatively high).