Sleep apnea causes this exact kind of sleep disorder.  Most MD's don't know it because they're ignorant of many symptoms. They think only overweight people with high blood pressure get sleep apnea.  But that's not true at all. Many women develop some form of sleep apnea after menopause.  The apnea repetitively awakens your brain, whereby you eventually wake up and can't get back to sleep.  I've experienced it for years before an NP finally figured out I might have a sleep breathing disorder and referred me for a sleep study. I told her my REM sleep was very fragmented with vivid dreams.  I have sleep apnea that causes chronic insomnia and have been using cpap machine.  It's not easy for me but helps somewhat.  Right now I'm trying an oral appliance from a sleep dentist clinic and it's helped somewhat but needs adjustment to further open my airway I think.