There are actually two forms of physical sleep apnea.
There is the too much fat/weight on the throat apnea.
But there is also a condition that the throat just collapses on itself, for no reason other than crappy genetics I guess. I suffer from that. It completely inhibits my breathing if my head is not tilted severely forward while I lay down. Like we're talking chin against my chest if I am on my back.
When I go to the dentist, I have been known to pass out because I can't breathe at all when laying on my back.
I stop breathing regularly, I snore like an angry bear, and I'm constantly choking in my sleep. All of this is odd because I am only 5'9, 155 pounds, and have a 15.5 inch neck.
It causes all kinds of other issues as well. Because I can't get into proper sleep from half waking up so many times thoughout the night, I tend to develop a form of "REM behavioral Disorder".
Basically... I punch my girlfriend in the head.
Luckily, she's a sweetie, and won't hit back because I am sleeping. But boy do I hear about it the next day.