I have had several of these overnight tests done ranging from 24hr ECG monitorings, to the oxygen level monitorings similar to what you mention Coolgran65. The equipment has never had any unpleasent smell to it, but sometimes I have not slept very well with it on due thefear of it becoming dislodged during the night and the test rendered null and void.
I am lucky enough to live near big new teaching hospital that is equiped with all the latest medical equipment, including high tech clinical measurement technology. I have had a full heart and lung profile done there a couple of years ago where the machine fully takes over and records your breathing for you, rather like a scanner, it tells you what its about to do and how, and for how long. Its used to diagnose COPD, Bronchiectosis, Asthma, and Sleep Apnoea. The test takes just over an hour on the machine. A marvelous machine although some stages you are totally out of control and unable to breathe in or out when you fell like you want to. I was diagnosed with the very early stages of Bronchiectosis, and prescribed some extra medication.