I once had a (crappy) job where I had to run a diesel truck inside a warehouse for an hour or two while I did my job.
They had a CO2 meter near where I was working that never registered a damn thing, meanwhile I had a sore throat for 2 months, and felt dizzy and nauseous for most of that time. The human resources at Iron Mountain didn't take my complaints seriously so I quit -- I should have taken the complaints to WSIB or the labour board, but I was too busy at the time and the job wasn't worth fighting for. Two days after I quit all my ailments were gone.
Anyway, to cut to the chase, I don't think that CO2 monitor was working properly and I am not sure if I would trust in such a device -- better to not take the risk by not placing yourself in a high CO2 environment, especially when sleeping and you may not notice any symptoms of CO2 poisoning.