Heritability of Insomnia Symptoms in Youth

1641 Heritability of Insomnia—Gehrman et al were done with adults and relied on individual self-report items rather than validated measures. As a result, findings from these studies may reflect the more general concept of “sleep problems” rather than insomnia per se. The lack of research in this area may reflect the traditional notion of insomnia as a symptom of another disorder rather than an independent clinical entity worthy of attention. For example, insomnia often occurs in conjunction with psychiatric disorders, most notably depression and generalized anxiety disorder, both of which have insomnia as a diagnostic criterion.8 As a criterion for each of these conditions, it is commonly felt that insomnia is simply a symptom (secondary to primary medical and/or psychiatric conditions). This said, it is now generally accepted within the sleep research community that insomnia can occur as an independent condition and that it is best construed as comorbid condition, one that has a bi-directional relationship with disorders such as depression and anxiety.9-11 There are a number of longitudinal studies demonstrating an increased risk of new-onset psychopathology over time in individuals with insomnia at baseline.12-15 In cases of remitted depression, insomnia often is a residual problem that confers increased risk for relapse.16,17 In cases where insomnia does resolve it often occurs as a prodromal symptom prior to relapse.18 Lastly, the neurobiology of sleep/wake regulation overlaps considerably with that for mood and anxiety,19 suggesting common mechanisms. These intricate interrelationships raise the question of the degree to which genetic effects are insomnia-specific vs. shared with these disorders. Several studies have now demonstrated that depression and generalized anxiety are different INTRODUCTION Insomnia is a highly prevalent sleep disorder, affecting 10% of the U.S. population on a chronic basis and 30% episodically.1 There is a large body of research on pharmacologic and cognitive behavioral treatments for insomnia, but investigations of the underlying mechanisms are relatively sparse. In particular, studies of genetic factors are lacking. While the heritability of most psychiatric disorders has been studied for over 20 years, our understanding of the role of genetics in insomnia remains limited. To date, several twin studies in adults have examined whether insomnia symptoms, broadly defined, are heritable traits. In one study, Heath and colleagues at the Australian Twin Registry collected sleep data from almost 4000 pairs of twins.2 The proportion of variance related to heritable factors (h2) was fairly consistent across questions ranging from sleep quality (h2 = 32%) to “anxious insomnia” (h2 = 36%). Subsequent studies generally yielded comparable estimates in the range of 20% to 40%,3-6 with the exception of one study that found a heritability of 64% for a single insomnia item.7 All of these studies HERITABILITY OF INSOMNIA SYMPTOMS IN YOUTH