Silly question perhaps, but I'm wondering

[Windswell]

Only once in 43 days of monitoring with an O2ring has my SpO2 dropped as low as 79. I'm normally in 92-97 range with occasional 3% drops and rare 4%drops bottoming out at 88. I look like a fit 72 y/o guy, 5'9", 153 lbs, active, windsurf, garden, kayak, but not REAL aerobically active. 
So, I tried to simulate my 79. I relaxed for a few minutes, O2 @96, then exhaled and held my breath which lasted about 38 seconds, but only drove my O2 down to 93. Then I relaxed O2 @95, and took a big deep breath, held it for 47 seconds and only drove it down to 92.

How does my O2 EVER drop as low as 79 ? Extended shallow breathing preceding the drop??

[KeepSmiling]

Are you on some type CPAP machine. If you are then something is not working. If not

Your original question, why this time.
- Had you taken any meds?
- Extra tired?
- Something you ate made you more sleepy.

Are you not using a machine and just depending on a tennis ball to keep you from going into a deep sleep so your oxygen will not drop?

That drop in oxygen you experienced is why it was drilled into me, even a catnap needs a machine.

I know someone who went in for a full night sleep study in a hospital. After a while was awoke to get a machine on because oxygen dropped to 86%. Early forty year old.

Very scary to me to see your results, unless you are supervised by a doctor.

[SevereApnea]

Greetings, to answer your question: because you were asleep.

Your chart says "fell asleep on my back, head up from reading" (Ooops!)

From your post you omitted to let us know if you were using the CPAP at the time.

[Caveat: I do not know how accurate your O2 ring is: my own Garmin watch O2 levels do NOT match my Nonin finger oximeter, so therefore I no longer look at that. The readings can be way off.]

If on the other hand you do trust your O2 readings then either (1) you were not using your CPAP when you fell asleep and you should have been, or (2) you were using your CPAP and you should be looking at and posting the corresponding OSCAR charts for comment.

Either way you should be thinking about a neck collar.  Neck flexion gets many people into trouble with Apnea. Think kinked garden hose. My own Sleep study showed me desaturating to 79% while asleep. (>>> grand mal seizure from sleep in 2019, 4 fractured vertebrae long story, hence now on CPAP).

But, like you, I am not able to get my 02 sats down below 90% while awake, (Nonin Oximeter connected to CPAP),despite long breath-holds and/or repeated long breath-holds till having to give up. The CO2 build forces one to breathe, while awake.

Now, while asleep this is a different story. This level of prolonged hypoxia combined with hypercapnia can be dangerous and should be treated/discussed with your doctor/PCP as a matter of priority.

I don't mean to alarm you, you raised the question after all, I am simply giving you my thoughts, I think this needs to be addressed.