The minimum criteria necessary to diagnose insomnia as a disorder include the complaint of inadequate nighttime sleep when one has the opportunity to be sleeping and the presence of associated negative daytime consequences. The specific sleep-related symptoms may involve difficulty falling asleep, difficulty remaining asleep, early morning awakening, and a sense that sleep is unrefreshing and nonrestorative. Although some insomnia subjects exclusively have difficulty falling asleep or middle-of-the-night awakenings, most commonly patients present with a combination of symptoms. Patients may report that it takes them hours to fall asleep, that they have multiple brief or prolonged nighttime awakenings, or experience very light sleep. Specific complaints may include a delayed sleep onset, short total time, and excessive awakenings and wake time during the night along with a general negative assessment of sleep quality.

Descriptions of daytime impairment or dysfunction almost invariably accompany the report of inadequate nighttime sleep (5). These daytime symptoms are a fundamental component of insomnia and frequently represent the prime motivation for patients to seek treatment for their sleep disturbance. Chronic insomnia patients tend to worry excessively about their difficulty sleeping and the impact it may have on their lives. Most insomnia patients complain of daytime fatigue that often is associated with low energy, decreased motivation, and a feeling of malaise. Cognitive symptoms may include poor attention, concentration, and memory. Patients with insomnia also may describe a greater tendency to make mistakes or be involved in accidents. They may complain of having a low mood or irritability. Although insomnia patients sometimes report daytime sleepiness, more commonly they describe an inability to nap in spite of a craving for sleep. The insomnia chief complaint may incorporate specific descriptions of school or work impairment, or difficulty in personal or social relationships. Patients also may present with accompanying physical concerns, such as headache, tension, and gastrointestinal symptoms.

Insomnia may be a relatively acute problem lasting several nights to a few weeks or may evolve into a chronic condition, which typically is defined as persisting for at least 1 month. Brief insomnia episodes often are attributable to situational crises and other apparent precipitants, such as acute health problems, medication effects, or schedule changes. Chronic insomnia generally involves additional perpetuating factors, such as psychologically conditioned arousal, poor sleep hygiene behaviors, or chronic comorbid health conditions. Chronic insomnia patients may offer histories of persistent difficulty with sleep onset or sleep maintenance; however, more typically they describe varying patterns over time. Patients also may report persistent nightly sleep difficulty, intermittent symptoms several nights a week, or occasional insomnia episodes with periods of normal sleep at other times. Episodes of insomnia may appear to occur randomly, although insomnia sufferers sometimes are able to associate their poor sleep with predictable associations, such as life stressors, work schedule patterns, and menstrual cycles.

There are multiple challenges in attempting to investigate the epidemiology of insomnia. The problem is complicated by the various possible definitions of insomnia, which fundamentally is a subjective complaint that can be viewed as a symptom or a disorder (2). Symptoms associated with insomnia may include specific representations of difficulty falling asleep or staying asleep, or more general descriptions of poor quality and nonrestorative sleep. These insomnia symptoms may exist independently, suggesting a primary insomnia disorder, or may be associated with an assortment of comorbid conditions, such as major depression. The fact that difficulty falling asleep or staying asleep are not necessarily perceived by individuals
as clinical problems further complicates insomnia prevalence estimates. Moreover, insomnia symptoms may vary in frequency, severity, and pattern over time. Accordingly, surveys asking whether people occasionally have difficulty with falling asleep, nighttime awakening, or early morning awakenings will suggest a very high prevalence of insomnia, while questionnaires targeting persistent sleep disturbance lasting at least several weeks and associated with daytime impairment will result in considerably lower rates of insomnia.

Ohayon has analyzed the variety of approaches to estimating the general population prevalence of insomnia and has defined four categories based upon increasingly specific criteria (6). The first and broadest group represents people acknowledging having at least one symptom of insomnia, such as difficulty initiating or maintaining sleep, early morning awakening, or nonrestorative sleep. About one-third of adults fall into this insomnia prevalence category. The second insomnia definition includes the presence of at least one of the nighttime symptoms, as well as an acknowledgement of daytime consequences associated with the nighttime sleep disturbance. Studies employing these more restrictive criteria find prevalence rates of 9% to 15%. Investigations specifically assessing dissatisfaction with sleep quality or quantity report rates ranging from 8% to 18%. Finally, a prevalence of approximately 6% has been found in studies employing the full Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) insomnia diagnosis criteria (7). The 2005 National Institutes of Health chronic insomnia State-of-the-Science conference statement summarized epidemiologic studies in concluding that approximately 30% of the population complains of sleep disruption, while about 10% also report associated daytime functional impairment (1).

Although people in all demographic groups may suffer with insomnia, it is clear that certain segments of the general population are at greater risk and have higher prevalence rates (6). Insomnia is more common in women than men, with a ratio of about 1.4. Among women, the prevalence is significantly greater during the postmenopausal years. Increasing age also is a major independent risk factor for both men and women. Other factors that appear to be associated an increased risk for insomnia include lower socioeconomic status, low levels of physical activity, and being single or widowed. Epidemiologic studies also confirm a significant interaction between insomnia symptoms and both psychiatric and medical disorders. One large-scale study employing specific insomnia criteria found a general population prevalence of 10%, of which 40% additionally met the criteria for a psychiatric disorder (8). Studies focusing on patients with psychiatric illnesses, particularly mood and anxiety disorders, generally find very high rates of insomnia. Similarly, patients with a variety of chronic health conditions complain of insomnia at higher rates than people without the conditions, while people with insomnia report higher rates of the health problems than those without insomnia. Specifically, insomnia subjects were more likely to report having heart disease, hypertension, neurologic disease, breathing problems, urinary problems, chronic pain, and gastrointestinal problems. The individuals with heart disease, cancer, hypertension, urinary problems, chronic pain, and gastrointestinal problems had higher rates of insomnia than those without these health problems (9).

The population of people with insomnia is quite diverse. They vary in nighttime and daytime symptoms, patterns of sleep disturbance, the course of insomnia over time, presence of comorbid conditions, precipitating circumstances, and demographic characteristics. The recognition of persistent insomnia with negative daytime consequences may be sufficient for the initiation of broad treatment approaches, although a more detailed classification may be important for certain clinical and
research activities, for diagnostic coding and billing purposes, and for developing more specific treatment plans for patients.

One major insomnia classification theme has been differentiating insomnia patients seeming to have an independent type of insomnia from those experiencing sleep disturbances in the context of another disorder (e.g., major depression, fibromyalgia). Previously these were termed primary insomnia and secondary insomnia. It had been assumed that with secondary insomnia an underlying condition caused the insomnia and that treatment of the other disorder should solve the insomnia problem. Primary insomnia continues to be viewed as a self-perpetuating insomnia syndrome involving conditioned arousal and excessive worry about sleep. Recently the term comorbid insomnia has been preferred as a replacement for the secondary insomnia concept, in part because of the absence of any evidence of mechanistic pathways whereby other disorders cause insomnia. The secondary insomnia assumptions also might lead to inadequate independent treatment of the insomnia if the therapeutic focus is entirely on the presumed underlying disorder (1). The comorbid insomnia conceptualization is supported by the epidemiologic evidence showing considerable overlap between persistent insomnia and various psychiatric, medical, and other sleep disorders. The majority of insomnia patients are classed in the comorbid insomnia category.

Th ree major nosologies that include classifications of sleep disorders have been developed and updated in recent decades. These include The International Classification of Sleep Disorders, 2nd Edition (ICSD-2) (5), DSM-IV (7), and the International Statistical Classification of Diseases and Related Health Problems, 9th Edition, Clinical Modification (ICD-9-CM) and 10th Edition (ICD-10) (10,11). These classification schemes have varying degrees of detail and number of specific insomnia diagnoses. The ICD classifications are very widely employed for coding and billing purposes, but have significant clinical and research limitations regarding insomnia diagnoses. The ICD systems recognize two broad categories of sleep disorders, organic and nonorganic (emotional), both of which include insomnia disorders.

The DSM-IV focuses on the classification of mental disorders and includes a section devoted to sleep disorders that is divided further into primary sleep disorders (dyssomnias and parasomnias), sleep disorders related to another mental disorder, sleep disorders related to a general medical condition, and substance-induced sleep disorders (7). Patients with insomnia may satisfy the criteria and be diagnosed with a disorder in one or more of these diagnostic categories. Among the dyssomnias is primary insomnia, here defined as a predominant complaint of difficulty initiating or maintaining sleep or nonrestorative sleep that has persisted for at least 1 month. The DSM-IV primary insomnia criteria also specify that there must be clinically significant distress or impairment in social, occupational, or other important areas of functioning, and that the sleep disturbance is not exclusively associated with substance use or another sleep, mental, or general medical condition. The DSM-IV primary insomnia discussion emphasizes the vicious cycle of negative conditioning and excessive arousal associated with attempts to sleep at night. Other major insomnia diagnostic options within the DSM-IV are insomnia related to another mental disorder and the sleep disorder due to a general medical condition, insomnia type. Within the DSM-IV framework, diagnosing a patient with one or more of the insomnia disorders is appropriate when the sleep disturbance warrants independent treatment. Proposed changes in the sleep disorders section of the future DSM-V include the elimination of the primary insomnia diagnosis in favor of an insomnia disorder with specifications of any comorbid psychiatric and medical conditions (12).

The most comprehensive and detailed overall categorization of sleep disorders and insomnia diagnoses in particular is the ICSD-2, which was most recently updated in 2005 (5). The ICSD-2 incorporates general insomnia criteria shared by the 11 possible specific insomnia diagnoses. It should be noted, however, that

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