I started having these last summer. At first I thought it was situational...since we had several stresses on vacation.

But this middle of the night awakening and anxiety continued.

I think it is related to age, and blood sugar. A dip in blood sugar does happen in the night and for some as we get older, the brain suffers. People who have a tendency to type II diabetes, often report disturbing dreams or nightmares in the wee hours.
When the blood sugar falls, there is a slight delay until the liver kicks in and releases more glucose for you. This will show up as a slightly elevated morning fasting glucose on testing. It is called the dawn phenomenon.

I keep a Zone nutrition bar by the bed and use it about once a week. It stops those feelings cold. I can recall one aunt in the past who had strange anxiety in the night when she was older. She used to obsess that mice got in her house and nibbled on match heads and would burn her down! (so she kept all the matches in glass jars). Then I saw an article in the newspaper about this VERY THING...as a cause of home fires! (Imagine how guilty we felt being critical of her obsession!)

I used to get anxious every night, but I have started a supplement called d-ribose, and it seems to block the anxiety and change my metabolism. I have also used l-tryptophan in 2 week bursts for this, and it is also helpful. I have hubby on it now, before our long trip by car...which seems to make him very anxious as well. (this trip is tough and requires we take alot of "things" because there is limited food choices up there, no electricity and no running water. --highly challenging as we are old, and have 3 old cats with special needs).