Sounds like the usual overnight oximeter test. I have had four overnight ones. It clips on to your finger and has something around wrist to keep it in place. They instruct one to keep track if one gets up, uses the bathroom etc. After, you give it back to them and they download the data for your report.
If you are in the hospital your oxygen may be monitored. For my two lung surgeries, my O2 was monitored and because it dropped significantly, they sent me home with 3 month prescription sleep O2 each time. I then had to do the overnight oximeter at three months to see if I needed to continue the night oxygen.