and has difficulty sleeping may then develop negative associations for sleep. Conditioned
arousal may then persist and lead to persiste nt insomnia. A similar course may develop in
the context of an acute psychological stress or a mental disorder. For instance, insomnia that
occurs during an episode of major depressive disorder can become a focus of attention,
with consequent negative conditioning, and pe rsist even after resolution of the depressive
episode. In some cases, insomnia may also have an insidious onse t without any identifi-
able precipitating factor. 
The course of insomnia may also be episodic,  with recurrent episodes of sleep difficul-
ties associated with the occurrence of stress ful events. Chronicity rates range from 45%
to 75% for follow-ups of 1–7 years. Even wh en the course of the insomnia has become
chronic, there is night-to -night variability in sleep patterns, with an occasional restful night’s
sleep interspersed with several nights of poor sleep. The characteristics of insomnia may
also change over time. Many individuals with insomnia have a history of “light” or easily
disturbed sleep prior to onset of more persistent sleep problems.
Insomnia complaints are mo re prevalent among middle-age and older adults. The type
of insomnia symptom changes as a function of age, with difficulties initiating sleep being
more common among young adults and proble ms maintaining sleep occurring more fre-
quently among middle-age and older individuals. 
Difficulties initiating and maintaining sleep can also occur in children and adolescents,
but there are more limited data on prevalence, risk factors, and comorbidity during these
developmental phases of the lifespan. Sleep diff iculties in childhood can result from con-
ditioning factors (e.g., a child who does not lear n to fall asleep or return to sleep without