For a small minority of people who actually seek help for a sleep disorder, the problem turns out to be Insufficient Sleep Syndrome. As the name implies, this is a disorder effecting people who simply do not get enough sleep at night.
The difference between people affected by this disorder and other people who are perpetually fatigued is that sufferers of Insufficient Sleep Syndrome don't realize that they aren't getting enough sleep at night. They aren't sure why they still feel tired after a night's sleep, and this is why they wind up seeking treatment.
Oftentimes this disorder can result from lifestyle choices. People start cutting back on sleep in order to get everything done that they need to do throughout the day, and in doing so they convince themselves that they don't need as much sleep as they thought. They get into the habit of getting far less sleep than their bodies actually require in order to function at full capacity.