Insomnia and related sleep disorders.

Insomnia may result from a variety of causes, including pathophysiology of the cardiorespiratory system, medical or psychiatric disorders, medications, substance abuse, and circadian rhythm disturbances. When these many causes are excluded, there remains an interesting group of patients whose disorder is characterized either by learned sleep-preventing associations or by a dissociation between physiologic measures and the subjective experience of sleep. One approach to understanding the latter group is to examine the patient's perception of the experience of being awake or asleep as well as the effects of hypnotic medication on this process.