Below is a graphic illustration of positional sleep apnea. My optimal pressure is 8.5 cm when sleeping on my tummy or either side. On my back, the sleep lab said they could not get me to breathe until they raised the pressure to 19 cm (ouch).
Yesterday was a very tiring day for my. In the morning I did a six-mile trail run that included a very steep and rugged 37-minute ascent. In the afternoon I worked in the lawn for two hours. After dinner, I did a little workout with weights. So when I hit the sack, I fell into a deep sleep immediately.
I have trained myself to sleep on my tummy (Falcon position) mainly and some on my sides. I always start out on my tummy. In the graph you can see a glorious CPAP result for the first four hours - no apneas, no hypopneas, no snores, one flow limitation, and very little pressure searching.

In hour five, I awoke with leg soreness, repositioned myself on my tummy still, and kicked in the ramp. Shortly thereafter I decided to get up and stretch my legs and take a pinkelpause. I returned to bed and decided to sleep on my side. Things went well again for about an hour.
Then at hour 5.5, I rolled onto my back, about five apneas occurred, and the machine slowly responded with higher pressure. Then I awoke, rolled onto my side, the apneas ceased, and the machine lowered the pressure.
Then just near the six-hour mark, I unknowingly rolled onto my back, the apnea "fun" began, and the machine begin raising the pressure. When the machine got the pressure to 14 cm, it appears to have the apneas under control.
I awoke at hour seven, realized what had happened and pressed the on/off button to drop the pressure. I lay there for a few minutes wide awake and then decided to get up.
That one series of apneas and the failure to sleep that last hour have left me with a little more edge than usual this morning. DNFWM!
That is a classic case of PSA with a wide range of pressure needs.
What is your PSA?
Regards,