I'm glad to find some new ideas about this form of insomnia, which I have been dealing with for years. It started about 15 years ago, when I would wake up automatically after about 4 hours of poor sleep and not be able to fall back to sleep until maybe an hour before I had to get up. FWIW my physician concluded it was a symptom of depression and prescribed Prozac plus a sedative, which seemed to work for the short term but was a failure long term.
Some of things I have done which seem to have helped quite a bit:
Avoidance of alcohol, caffeine, sugar.
Vitamin/ mineral supplements
Daily exercise and stretching
Exploration of psychological issues related to my FOO. Realization of these problems seems to have calmed me down quite a bit. Maybe I'm not so stressed by the search for my "real family" the way I was during my early adulthood.
It now seems to be a more seasonal short term thing. The change of seasons seems to throw me "out of whack" for awhile. I often have a bout of insomnia going into late Fall. Maybe it's intensified by realizing the holidays are coming and I probably have to endure the FOO again