This past week, I had to travel to Philadelphia to see a medical specialist regarding an unrelated ailment. I took my APAP with me to test what a change of altitude would have on my sleeping.
On 31 Jan, I was at Salt Lake City at an elevation 4200 feet. That night, my AHI was 8+, not statistically different from my exeprience at 6000 feet.
On 1 Feb, I was in Philadelphia, at an elevation of 39 feet. My AHI was 2.5, the best AHI I ever had on APAP. I had, for the first time, absolutely no CA's. At no time during the night did my hourly AHI ever exceed 5.
On 2 Feb, I was also in Philadelphia. My AHI was 2.7 also well below any data that I had at home. I had a lot of problems this night with leg cramps and was awake and walking around for about five minutes each hour. This was probably due to dehydration. Yet, this did not trigger apneas that I would experience at 6000 feet. I had very minimal number of CA's.
On 3 Feb, I was again in Salt Lake City. My AHI was 4.5: my CA's were again nornal.
Finally, last night, 4 Feb, I was again at home and had an AHI of 8.6 with normal CA's.
The time is not long enough to definitively say that sleeping at near sea level was much better for my apneas but there is a clear indication to this effect. Surprisingly, I noted that it seemed harder to breath against my pressure settings at low altitude than at home.
This is just one data point.