I've sometimes wondered if what I thought was insomnia was periods of very light sleep between deeper sleep, but seemed to be a continuous stretch of being awake. Scientists have also wondered.
From a Gizmodo piece on a study published last year in New Scientist:
When they used conventional methods of analyzing the brain wave patterns of sleepers, it appeared that these pseudoinsomniacs were no different than any other sleeper. But then they tried something new. ...
And that's when they began to see a pattern — a pattern that suggested these peoples' sleep cycles were being interrupted by brain wave patterns associated with fear, anxiety, and wakefulness.
From the New Scientist article:
In other words, their problem is not so much that they don't sleep but that, asleep or not, their brains are never quite off. "It's the major hypothesis of what insomnia's about," he says.
Do You Feel Like You're Not Sleeping, Even When You Are?
The New Scientist article is behind a pay wall. It's expensive.
Should you wish to discuss insomnia cures, I suggest contributing to one of the existing threads:
Insomnia Cure with 80%+ Success Rate for Me