The essential feature of this disorder is the occurrence of insomnia that occurs exclusively
during the course of a mental disorder, and is judged to be caused by that disorder. The
insomnia is of sufficient severity to cause distress or to require separate treatment. This
diagnosis is not used to explain insomnia that has a course independent of the associated
mental disorder, as is not routinely made in individuals with the “usual” severity of sleep
symptoms for an associated mental disorder