It could possibly be some form of dysautonomia. I had similar nausea/dizziness issues, although mine were predominantly, but not exclusively, at night. I figured it out myself, and it took about ten doctors before I found someone who actually knew anything about it and could help. Essentially it came down to an overstimulated vagus nerve and really unstable blood pressure. There is medication that can help, notably Florinef (fludrocortisone). I also consume massive quantities of sodium -- probably 4-6 SaltStick capsules per day, plus an Osmo Preload every day -- as well as lots of water. Try to find a specialist before you do that, though. It's usually a cardiologist or neurologist. David Cannom in the Los Angeles area is good if that's where you are.

P.S.: Don't be alarmed by the Parkinson's part on the web page I linked. People with Parkinson's often have dysautonomia, but very few people with dysautonomia have Parkinson's.