Insomnia is a very common disorder. It's hard to diagnose and sleep studies are often not helpful. Primary insomnia (PI) means the insomnia is not caused by another disorder. PI is also common. Studies have not shown that patients with PI suffer any objective neurocognitive (thinking performance) deficits.
Here is an interesting study in the Journal Sleep. Dr. Edinger discusses subtypes of insomniac - those who sleep a normal amount versus those who are short sleepers, and those who are alert in the daytime vs those who are sleepy. In this study, sleep duration was measured with overnight sleep studies. Level of daytime sleepiness was measured with a daytime sleepiness test.
Physiologic hyperarousal is a common problem with insomniacs. This refers to elevated nervous system activity - think adrenaline. This can disrupt nighttime sleep but also allow sleep deprived people to be alert in the daytime. The results of this study showed that insomniacs with hyperarousal had lower neurocognitive test scores. Of note, people who were sleepy but not insomniacs also had lower neurocognitive test scores.
As mentioned in the commentary on this article, this study has real world implications. Insomniacs with hyperarousal may respond, at least in part, to biological treatments to lessen hyperarousal. And those insomniacs that do not have hyperarousal may respond more to psychological treatments like cognitive behavioral therapy.