368 Sleep-Wake Disorders
Substance/medication-induced sl eep disorder, insomnia type. Substance/medication-
induced sleep disorder, insomnia type, is distinguished from insomnia disorder by the fact
that a substance (i.e., a drug of abuse, a medica tion, or exposure to a toxin) is judged to be
etiologically related to the insomnia (see  “Substance/Medicatio n-Induced Sleep Disor-
der” later in this chapter). Fo r example, insomnia occurring only in the context of heavy
coffee consumption would be diagnosed as caffeine-induced sleep disorder, insomnia
type, with onset during intoxication.
Comorbidity
Insomnia is a common comorbidity of many medical conditions, including diabetes, cor-
onary heart disease, chronic obstructive pulm onary disease, arthritis, fibromyalgia, and
other chronic pain conditions. The risk relati onship appears to be bidirectional: insomnia
increases the risk of medical conditions, and medical problems increase the risk of insom-
nia. The direction of the relationship is not always clear and may change over time; for this
reason, comorbid insomnia is the preferred terminology in th e presence of coexisting in-
somnia with another medical co ndition (or mental disorder).
Individuals with insomnia disorder frequent ly have a comorbid mental disorder, par-
ticularly bipolar, depressive, and anxiety diso rders. Persistent insomnia represents a risk
factor or an early symptom of subsequent bi polar, depressive, anxi ety, and substance use
disorders. Individuals with insomnia may misuse medicati ons or alcohol to help with
nighttime sleep, anxiolytics to combat tension or anxiety, and caffeine or other stimulants
to combat excessive fatigue. In addition to worsening the insomnia, this type of substance