I had one of these done 5 years ago and am going in for another next week.
They found that my brain waves kept going into a lighter stage of sleep due to "alpha intrusions" which is common in fibro. You basically don't get into the deeper levels of sleep. My test showed that my brain was technically awake hundreds of times through the night. They labeled it chronic insomnia and I haven't found anything that helps. They mentioned mild apnea for me as well. I used the CPAP for 3 months I believe before I got annoyed that it wasn't helping my symptoms and stopped.