that must be addressed in treatment planni ng and management. Fu rthermore, persistent
sleep disturbances (both insomn ia and excessive sleepiness) ar e established risk factors for
the subsequent development of mental illne sses and substance use disorders. They may
also represent a prodromal expression of an episode of mental illness, allowing the possi-
bility of early intervention to preemp t or to attenuate a full-blown episode.
The differential diagnosis of sleep-wake co mplaints necessitates a multidimensional
approach, with consideration of  possibly coexisting medical and neurological conditions.
Coexisting clinical conditions are the rule, no t the exception. Sleep disturbances furnish a
clinically useful indicator of medical and ne urological conditions that often coexist with
depression and other common mental disord ers. Prominent among these comorbidities
are breathing-related sleep disorders, disord ers of the heart and lungs (e.g., congestive
heart failure, chronic obstructive pulmonary disease), neurodegenerative disorders (e.g.,
Alzheimer’s disease), and disorders of the musculoskeletal system (e.g., osteoarthritis).
These disorders not only may disturb sleep but also may th emselves be worsened during
sleep (e.g., prolonged apneas or electrocardi ographic arrhythmias during REM sleep; con-
fusional arousals in patients with dementing illness; seizures in persons with complex
partial seizures). REM sleep behavior disorder is often an early indicator of neurodegen-
erative disorders (alpha synucleinopathies) li ke Parkinson’s disease. For all of these rea-
sons—related to differential diagnosis, clinic al comorbidity, and fa cilitation of treatment
planning—sleep disorders are included in DSM-5.