Jack D. Edinger, Meir H. Kryger and Thomas Roth

Overview

The term insomnia refers to a condition characterized by difficulties initiating and/or maintaining sleep, accompanied by clinically significant daytime impairment or distress related to the ongoing sleep difficulties. Insomnia may occur as an independent sleep disorder, or it may evolve as a symptom of a medical condition, psychiatric illness, or other sleep disorder. However, even when insomnia initially arises as a symptom of another disorder, it often develops partial or total independence over time and eventually warrants separate clinical attention. Given this observation, it is best to use the term comorbid insomnia when prominent, clinically significant insomnia symptoms are observed concurrent to another medical, psychiatric, or sleep disorder.

The sleep and waking symptoms of insomnia are varied and can change over time. Insomnia sufferers may come to medical attention solely with sleep onset difficulties or sleep maintenance problems, or they may be seen with a combination of these. Unrefreshing or nonrestorative sleep may accompany these other symptoms but is relatively rare as an isolated symptom and may connote another underlying primary sleep disorder (e.g., obstructive sleep apnea). Isolated sleep-onset complaints are relatively infrequent but occur most commonly in younger age groups, whereas sleep maintenance and mixed onset and maintenance problems are more common presentations of insomnia, particularly in middle-aged and older adults. Yet insomnia symptoms are not necessarily stable within individuals but may vary over time, in that patients who come to medical attention with isolated sleep onset difficulties may later develop sleep maintenance difficulties or a mixture of sleep onset and maintenance problems. Common daytime consequences of insomnia include impaired cognitive function, fatigue or tiredness, feelings of sleepiness, reduced motivation and initiative, and depressed mood or irritability. In addition, adults may display increased proneness for accidents at work or while driving, whereas poor school performance and behavioral problems, such as hyperactivity, are common among school-age children and adolescents.

Many patients with insomnia come to sleep disorder centers for evaluation and management. For a majority of such patients, a clinical interview that includes a thorough sleep history and mental status along with a medical history and examination provides sufficient information for case conceptualization and treatment planning. Polysomnography (PSG) is not routinely indicated for the evaluation of insomnia complaints, unless symptoms suggest another primary sleep disorder, such as obstructive sleep apnea (OSA) or periodic limb movement disorder (PLMD), which requires PSG for diagnosis and subsequent management.

Classification

Historically, a great deal of controversy and confusion has accompanied the diagnostic classification of insomnia because several nosologies use differing approaches to insomnia classification or subtyping. Included among these are the insomnia classification systems contained in the International Classification of Diseases (ICD), the diagnostic and statistical manuals of the American Psychiatric Association (APA; DSM-III-R, DSM-IV, and DSM-IV-TR), and the previous and current versions of the International Classification of Sleep Disorders (ICSD, ICSD-R, and ICSD-2, respectively). These three nosologic systems differ markedly in regard to the numbers and types of insomnia diagnoses they describe. As a consequence, application of these different diagnostic schemes to the same insomnia patient population provides notably discordant diagnostic results.

In addition, the putative distinction between primary and secondary insomnias propagated by many past and current insomnia classification schemes had complicated insomnia diagnostic practice as well. Historically, the term primary insomnia has been used to connote a form of insomnia that exists as an independent sleep disorder, whereas the term secondary insomnia has been used to connote the symptom of insomnia that arises from a coexisting medical, psychiatric, or sleep disorder. However, both research and clinical experience have shown that patients assigned primary and secondary insomnia diagnoses often have myriad overlapping symptoms that make it difficult, if not impossible, to reliably discriminate between them. Furthermore, although some patients may have a true secondary insomnia that results from another coexisting disorder, others may have insomnia that is partially related or unrelated to the putative primary coexisting condition. Unfortunately, we have no reliable methods or assays for differentiating among these distinctive “secondary” subtypes. Hence, as recommended by the 2005 National Institutes of Health consensus statement concerning the manifestations and management of insomnia, it seems best to use the more globally descriptive term, comorbid insomnia for forms of insomnia accompanied by another primary sleep disorder or sleep-disruptive medical or psychiatric condition.

Considerable collaboration has occurred between the organizations developing the insomnia classification scheme included in subsequent editions of the APA’s Diagnostic and Statistical Manual (i.e., the DSM-5) and the ICSD-3. This collaboration has resulted in much greater concordance between the insomnia classification schemes included in these two systems, and it has eliminated the primary versus secondary insomnia distinction in favor of the single diagnosis of insomnia disorder, which can be applied to all insomnia patients with and without sleep-disruptive comorbidities. Of course, both systems advocate the assignment of comorbid diagnoses when they apply, but they eliminate the need to ascertain cause/effect relationships between insomnia symptoms and coexisting conditions, thus simplifying the overall diagnostic process.

Epidemiology

Approximately 30% to 40% of adults have some degree of disturbed sleep during any given year. Estimates of the prevalence of those who meet formal criteria for an insomnia diagnosis vary from between 4% and 22%, depending upon the specific diagnostic criteria used (Fig. 10-1). Insomnia is more prevalent in women than in men, but the propensity to develop insomnia increases in both genders with advancing age, largely because of the increased occurrence of sleep-disruptive comorbidities that comes with aging (Fig. 10-2). Isolated sleep maintenance complaints are more common than isolated sleep onset complaints, although a sizeable percentage of insomnia sufferers have mixed complaints (Fig. 10-3). Insomnia symptoms may vary over time, but a substantial proportion of those who develop insomnia have persistent sleep difficulties that fail to remit without intervention.

Pathophysiology of Insomnia

Much about the etiology and pathophysiology of insomnia remains unknown. However, it is recognized that various factors may contribute to the development of insomnia, including comorbid sleep disorders, psychiatric and medical illnesses, certain medications and illicit substances, sleep-disruptive environmental circumstances, and a range of psychologic and behavioral factors. Although no single model explains all forms of insomnia, some theories and models are helpful. Spielman’s 3-P model highlights the roles of predisposing, precipitating, and perpetuating factors and is useful in helping understand the evolution of insomnia. According to this model, insomnia propensity may exist as a latent trait in predisposed or vulnerable individuals but typically does not become manifest until precipitating circumstances, such as a serious illness or stressful life event, push the individual over the insomnia threshold. Once the insomnia is present, it evolves from an acute to a more chronic problem, as the individual develops maladaptive responses to the sleep-wake disturbance that only serve to perpetuate it (Fig. 10-4). This model serves as a useful heuristic for understanding the evolution of chronic insomnia and has served as the basis for more complex theoretic models proposed for explaining the development and nature of insomnia complaints (Fig. 10-5).

One of the underlying assumptions of current theories is that insomnia patients are in a state of hyperarousal over the 24-hour period and that this propensity toward hyperarousal leads patients to develop sleep disturbances when stressed. This hyperarousal can be viewed as a predisposing factor for insomnia and may be the basis for some individuals’ development of insomnia in response to certain medical conditions, such as chronic obstructive pulmonary disease (COPD). However, hyperarousal may also serve a perpetuating role that makes sleep difficult throughout the 24-hour day. Numerous studies have supported the presence of physiologic hyperarousal among insomnia sufferers. For example, imaging studies conducted with positron emission tomography show that various brain areas of insomnia patients show less deactivation during non–rapid eye movement (NREM) sleep than do similar brain areas in normal sleepers (Fig. 10-6). Other research also supports the notion that the hyperarousal state results in measurable changes in physiologic systems other than the central nervous system (Fig. 10-7).

Types of Insomnia

The insomnia features described in Box 10-1 are found in three groups of patients (Fig. 10-8): 1) patients in whom a medical or psychiatric condition coexists with the insomnia (comorbid insomnia); 2) patients in whom the primary sleep disorder may include symptoms of insomnia (primary sleep disorders); and 3) patients in whom insomnia exists in the absence of a psychiatric, medical, or other primary sleep disorder (isolated insomnia disorder).

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