I can only start a single measurement manually, not a constant measurement overnight. And such watches are not medical devices. The best thing is to borrow an oximeter from your family doctor, who will also give you competent advice. Or order a Contec oximeter from Aliexpress - at least they have FDA approval.

by the way, some Garmin parts can do this, you can set whether only during sleep or the whole day, e.g. to measure the acclimatisation at high altitudes ...

Not in my opinion. There is no setting where you could specify this; seems to be an on-demand measurement.