In a recent issue of the journal, Sleep, a report was published which found that chronic insomniacs who woke up approximately 30 times a night on average was actually linked to respiratory problems. None of these chronic insomniacs had a any idea that their sleeping troubles was linked to a respiratory problem.
The article does mention that this study is in no way conclusive, because it included just 20 people and had no control group of normal sleepers to compare with. This article presents an excellent example of a study that wasn't necessarily reliable, but was valid. It presented consistent information among the few participants it did have, but because of the small group of participants and the lack of a control group the study was unreliable.
As the article mentions, although this study is not reliable or conclusive, its findings shouldn't be ignored. A follow-up study would provide an opportunity to uncover the reasons for these chronic insomniacs inability to sleep in a more reliable, controlled experiment.