A sleep complaint may arise from a primary sleep disorder. That is, there may be a disorder of normative sleep physiology. The disorders of narcolepsy, obstructive sleep apnea syndrome, and REM behavior disorder are examples of primary sleep disorders. Sleep problems also may arise secondary to a disorder that occurs during wakefulness, or there may be an exacerbation of such a disorder during sleep. Insomnia secondary to depression is considered to be a secondary sleep disorder.
Nocturnal asthma is an example of a waking disorder which is typically exacerbated by sleep.
Drugs used to treat waking disorders can also cause sleep difficulties. For example, respiratory stimulants used to treat chronic obstructive pulmonary disease and H1 antihistamines used to treat allergies may result in complaints of insomnia and daytime sleepiness, respectively. Disruption of the sleep-waking schedule can also lead to sleep complaints. Finally, it is important to recognize that sleep problems can be due to behavioral maladaptation. Knowledge of behavioral abnormalities are seen as critical in the evaluation of insomnia.