Patients with chronic insomnia can have problems falling asleep and staying asleep. For those that wake up at night, it can be difficult to know what wakes them up - after all, they were asleep! I've followed Dr. Krakow's work over the years, and he has a practice and does research over in New Mexico. His work has provided evidence that some "insomnia" patients actually have obstructive sleep apnea (OSA), and that treating the OSA can reduce the insomnia.
Dr. Krakow et al did a study published in the Journal Sleep. He did sleep studies on 20 patients who presented only with insomnia. These patients did not have other symptoms of OSA, although half of them snored. The mean body mass index was <25 and most were tired but not sleepy. The results showed that 11 patients had OSA and another 7 patients had a milder version of OSA called upper airway resistance syndrome. So that means 19/20 patients had some form of sleep disordered breathing. What we don't know is how these patients developed chronic insomnia. But the results help confirm what I see clinically quite often - that insomnia is not just a psychological problem but can be triggered by medical disorders like OSA.