Insomnia can be classified as primary or secondary (Table 1TABLE 1
Classification of Adult Insomnia.
).3 The pathogenesis of primary insomnia is unknown, but available evidence suggests a state of hyperarousal. As compared with controls, patients with insomnia show increased global cerebral glucose metabolism on positron-emission tomography when awake and asleep, increased beta activity and decreased theta and delta activity on electroencephalography during sleep, an increased 24-hour metabolic rate, and higher levels of secretion of adrenocorticotropic hormone and cortisol.8
Insomnia secondary to other causes is more common than primary insomnia (Table 1) and must be excluded or treated before making a diagnosis of primary insomnia. If insomnia persists despite treatment of secondary causes, then therapy for primary insomnia should be instituted. Several causes of the disorder may be present in a single patient. Circadian-rhythm disorders, such as shift-work sleep disorder and delayed-sleep-phase syndrome (a delay in the sleep period of more than two hours relative to conventional times), and voluntary insufficient sleep syndrome should be considered in the differential diagnosis, but these are not considered forms of insomnia.