SpO2 readings are only once per hour at night

I purchased this watch specifically for continuous SPO2 monitoring at night, to confirm treatments for sleep apnea are working. Much to my dismay, after turning on the setting for continuous monitoring at night, on sync with Garmin Connect the next morning, the graph shows readings only once per hour. Even Oura ring, which is “supposedly” less accurate, shows readings every 15 minutes.

To make matters worse, I am reading here that these watches are the least accurate for measuring SpO2 in the industry. After a single night of use and seeing readings that go down to 84% ( with my appliance), I wonder whether or not this can be used at all for sleep apnea. 

Does this device really not measure continuously? If it does, then why does Garmin show only one reading per hour? Before I send this watch back and go with another brand, I want to at least give the community a chance to provide some feedback. Overall, extremely disappointed.