Many questions remain unanswered with regard to our understanding of insomnia. Although it is generally believed that 10% to 15% of the adult population suffers from chronic insomnia, and an additional 25% to 35% have transient or occasional insomnia, prevalence estimates vary because of inconsistent definitions and diagnostic criteria. The elderly in particular are affected by insomnia, and it has been shown that women are more likely to have sleep difficulties than men. Although insomnia can be a primary condition, and can coexist with other disorders or be considered secondary to these disorders, the mechanisms producing it are not clearly defined. Additionally, the relationship between insomnia and other disease states is not always clear because it is often not possible to determine the cause-and-effect relationship between disorders. Epidemiologic studies show that abnormal sleep patterns predict lower life expectancy, and that people with insomnia are more likely to develop affective disorders, substance abuse, and other adverse health outcomes. This article will provide an overview of insomnia, its prevalence and epidemiology, and guidelines for clinical assessment.
(Am J Manag Care. 2006;12:S214-S220)
Sleep accounts for one third of human life, yet scientific inquiry in this area is limited compared with other aspects of neuroscience.1 Additionally, studies suggest that poor sleep contributes to ill health. Epidemiologic studies show that abnormal sleep patterns predict lower life expectancy,2 and that insomnia frequently co-occurs with affective disorders, substance abuse, and other physical and psychological comorbidities.3,4 However, research into the relationships between these findings is sorely lacking.
The definition of insomnia is a complaint of disturbed sleep, manifested as difficulties in sleep initiation or sleep maintenance, and/or as early awakenings. Many sources also add the presence of associated daytime impairments, such as fatigue, irritability, decreased memory and concentration, and pervasive malaise affecting many aspects of daytime functioning.5 Although all definitions of insomnia rely on its symptomatic presentation, a standard diagnostic definition does not exist. Three separate texts present diagnostic criteria for insomnia: The Diagnostic and Statistical Manual of Mental Disorders (DSM)6; The International Classification of Sleep Disorders7; and The ICD-10 Classification of Mental and Behavioural Disorders.8 Some definitions are based solely on reports of nocturnal sleep disturbances,9 whereas others include features such as associated daytime impairment (eg, fatigue, irritability, or reduced memory or concentration),10 self-reported sleep dissatisfaction,11 or other criteria.6,12-15
Attempts have been made to subtype insomnia. One method is based on duration of symptoms, identifying insomnia as either chronic (long-term) or acute (transient). A 2005 National Institutes of Health (NIH) State-of-the-Science statement pointed out that time periods of various durations have been used to define chronic insomnia, ranging from 30 days to 6 months.5 The transient/ chronic distinction can be clinically relevant, inasmuch as transient insomnias often result from specific environmental or social events, such as shift work, death of a loved one, air travel, and noise, and may be more appropriately managed by addressing these stressors and by managing the insomnia directly (and often prophylactically). On the other hand, chronic insomnia may be more often related to intrinsic sleep disorders, primary insomnia, or chronic medical and psychiatric conditions, and may require a more extensive evaluation (including assessment of comorbid conditions) in order to delineate appropriate treatment. However, it should be stressed that the relationships between insomnia duration, etiology, and evaluation implications have not been well investigated.
Insomnia can also be classified on the basis of etiology into primary and secondary subtypes. The term primary indicates that the insomnia is not caused by any known physical or mental condition but is characterized by a consistent set of symptoms, a defined disease course, and a general responsiveness to treatment.16,17 Although the etiology of primary insomnia has yet to be clarified, recent research implicates endocrine, neurologic, and behavioral factors as contributing to its pathogenesis.18-20 It is estimated that among patients diagnosed with insomnia, 25% to 30% suffer from primary insomnia.6,21 Secondary insomnia, in contrast, has been defined historically as insomnia resulting from other medical and psychiatric illnesses, medication use, or other primary sleep disorders.5,22 The 2005 NIH State-of-the-Science statement, however, has suggested the use of the term comorbid insomnia, instead of secondary insomnia, based on a limited level of understanding of the causal relationships which may exist between insomnia and coexisting disorders. Conceivably, primary insomnia could coexist as an independent entity in the context of another disorder, as opposed to being caused by it.5
Epidemiology and Natural History of Insomnia
Estimates of the prevalence of insomnia are variable because definitions and diagnostic criteria for insomnia are inconsistent. In addition, the use of baseline and follow-up assessments to establish incidence and remission rates can be problematic because of the wide spectrum of insomnia duration (eg, a positive finding of insomnia at baseline and 1-year follow-up may reflect unremitting chronic insomnia or 2 episodes of transient insomnia).23 Given these limitations, it is generally believed that 10% to 15% of adults suffer from chronic insomnia,24 usually regarded as a persistent insomnia lasting more than 1 month, and an additional one third have transient or occasional insomnia.25
The elderly in particular are affected, with an estimated prevalence ranging from 13% to 47%.22,26-31 The National Institute on Aging's Established Populations for Epidemiologic Studies of the Elderly (EPESE) 3-year longitudinal study showed that 42% of community- dwelling seniors who participated in the survey had difficulty falling and staying asleep.26,32 Sleep difficulties were more prevalent among seniors with physical disability, depressed mood, respiratory symptoms, or fair-to-poor perceived health and among those using anxiolytic and barbiturate prescription medication. At the 3-year follow-up of EPESE, Foley et al estimated incidence and remission rates for insomnia in more than 6000 participants of the original survey.32 Among 4956 participants who had no symptoms of insomnia at baseline, nearly 15% reported symptoms at the 3-year follow-up, suggesting an annual incidence of 5%.
In the same study, for about 15% of participants, insomnia symptoms resolved each year. Extrapolating these findings to the general population, the authors estimated that 8 million elderly persons nationwide have insomnia on any given day, more than 1 million new cases of insomnia develop each year, and symptoms resolve in nearly 1.3 million elderly persons annually.32 Disturbed sleep is also associated with impairments in memory and attention, and can be misinterpreted as signs of dementia in the elderly.33
Although most epidemiologic studies indicate that women are more likely to have sleep difficulties than men,27,34 the EPESE study reported comparable rates in both sexes. The exception to this parity occurred in patients 85 years or older, in which the prevalence was higher among men.32 The EPESE study also showed that women were less likely to achieve remission (46% of women vs 52% of men), suggesting the higher prevalence in women reported in other epidemiologic studies may indicate fewer remissions in women, not more new cases.32 This hypothesis was supported by findings from the Cardiovascular Health Study of 2005, which reported that women with insomnia were less likely than men to remit.35
In addition to the EPESE study of elderly patients, several other longitudinal studies have helped clarify the natural history of chronic insomnia. Breslau et al conducted a baseline assessment and 3.5-year follow-up of 1200 young adults (aged 21-30 years) drawn randomly from a health maintenance organization database. The lifetime prevalence of insomnia in this population was 24.6% and was slightly higher in women than men (26.7% and 21.4%, respectively). The 3.5-year incidence of new insomnia among subjects with no insomnia at baseline was 14.8% for women and 10.6% for men; slightly less than the incidence rates reported in the EPESE study.36
In a study of 521 healthy middle-aged women near menopause presenting at a clinic, Owens and Matthews found a very high prevalence (42%) of self-reported sleep difficulties. Among those reporting sleep problems, the most prevalent complaint was awakening during the night (reported by 92%), followed by earlier-than-desired awakening (59%) and trouble falling asleep (49%). A cross-sectional analysis failed to identify significant associations between pre-, peri-, or postmenopausal status and general or specific sleep complaints. However, among the subset of women who were premenopausal at baseline and postmenopausal and not using hormone replacement therapy at follow-up, a higher proportion reported sleep difficulties at the postmenopausal than at the premenopausal assessment.37
Hohagen et al conducted a study of 2512 patients aged 18 to 65 years presenting to primary care clinics in Germany; a baseline assessment identified 18.7% with severe insomnia (DSM-III-R criteria), 12.2% with moderate insomnia (DSM-III-R criteria, without impairment of daytime functioning), and 15% with mild insomnia (occasional difficulties in initiating and maintaining sleep). Follow-up assessments were conducted in patients reporting baseline insomnia at 4 months and 2 years. At baseline, mild insomnia was more prevalent among men, but severe insomnia was more common among women by a nearly 2:1 margin (65% vs 35%). More than two thirds of patients with severe insomnia at baseline reported a disease duration of 1 year or more.38