
Insomnia, the subjective sense of having inadequate quantity orquality of sleep, may result from a variety of causes. In itschronic form, such etiologies may include psychiatric disorderssuch as depression or anxiety, medical illnesses, medications,substance abuse, circadian dysrhythmias and pathophysiologiesintrinsic to sleep such as sleep apnea or periodic movement disorder.When these conditions have been ruled out, however, there remaintwo very interesting groups that at this time are best understoodin psychophysiological terms.