Sleep Symptoms Predict the Development of the Metabolic Syndrome

THE CLUSTERING OF CARDIOVASCULAR RISK FACTORS KNOWN AS THE METABOLIC SYNDROME IS STRONGLY AND PROSPECTIVELY LINKED WITH incident cardiovascular events, diabetes, and mortality.1 Given that prevalence rates of the metabolic syndrome are estimated at 20% in the adult population,2 identifying modifiable risk factors associated with the development of the metabolic syndrome is of critical public health importance. Several prospective studies have documented an independent relationship between sleep disturbances, including sleep disordered breathing (SDB) and sleep duration, and increased risk of developing individual components of the metabolic syndrome, including obesity, hypertension, glucose intolerance, and diabetes.3–8 Additionally, a handful of cross-sectional studies have shown that a broader range of self-reported sleep disturbances, including snoring, sleep duration, difficulty initiating and maintaining sleep, and poor sleep quality, as well as polysomnographically assessed sleep architecture are associated with prevalent metabolic syndrome.9–13 However, to our knowledge, no study to date has prospectively examined the relationship between sleep disturbances that commonly present in clinical practice and the development of the metabolic syndrome. Given evidence that there may be added prognostic value of the metabolic syndrome, over and above its individual components, and the inability of cross-sectional studies to support the proposed direction of causality between sleep disturbances and metabolic dysregulation, prospective evidence is clearly needed to examine whether sleep disturbances predict the development of the metabolic syndrome. Most epidemiologic studies of sleep and CVD risk have examined sleep symptoms rather than sleep disorders identified by diagnostic criteria, such as insomnia or obstructive sleep apnea syndrome (OSAS), which are the two most common adult sleep disorders.14 Understanding the relative impact of sleep symptoms versus sleep disorders may have important public health implications given that symptoms are much more prevalent than disorders,14 and the putative mechanisms linking sleep with cardiometabolic consequences may differ for symptoms and disorders. Moreover, there is considerable overlap in SDB and insomnia symptoms. Thus, to better understand the pathophysiology underlying links between sleep symptoms and cardiometabolic risk, it is critical to examine the independent effects of respective insomnia and SDB symptoms as well as more formally defined disorders. The present study investigated the degree to which insomnia or SDB predicted the development of the metabolic syndrome and its component factors (hyperglycemia, central adiposity, hypertension, hypertriglyceridemia, and low high density lipoprotein cholesterol) over a 3-year period in a community sample. We used two different case definitions for both insomnia and SDB. For insomnia, we examined individual insomnia symptoms as well as insomnia syndrome, which included a sleep complaint along with reported daytime impairment. For SDB, we examined whether snoring, the most common symptom of OSA, predicted the development of the metabolic syndrome, independent of apnea-hypopnea index, a physiological indicator of OSA. Given racial and gender differences in the prevalence of sleep disorders and the metabolic syndrome, for significant effects, we explored whether relationships between sleep symptoms and the metabolic syndrome differed among men and women and among blacks and whites.