Insomnia

Insomnia is not a single disorder, but rather a broad term used to indicate difficulty falling asleep or staying asleep for as long as one wishes. Dement considers it more a symptom than a disease. It is difficult for doctors to get much data on insomnia, because it is a self-diagnosed problem, though it is linked to other disorders, especially stress, psychological and emotional problems, and gastro-intestinal reflux. Insomnia's main danger is that it leads to uncontrollable daytime drowsiness, when the insomniac's sleep debt must finally be "paid."

Source:

Dement, The Promise of Sleep