A Longitudinal Twin Study of Insomnia Symptoms in Adults

Objective: Twin modeling was used to conduct a genetically informative longitudinal analysis of insomnia symptoms in both sexes. Method: Data from the Virginia Adult Twin Studies of Psychiatric and Substance Use Disorders (n = 7,500) were used. Past-month insomnia symptoms were assessed at two time points with the shortened version of the Symptom Checklist-90. A composite score for the insomnia items (trouble falling asleep, restless or disturbed sleep, early morning awakenings) was computed. Twin modeling on the composite score was conducted in OpenMx to decompose the phenotypic variance, to examine the longitudinal stability of etiologic influences on insomnia symptoms, and to test for sex differences. Results: Insomnia symptoms were most commonly endorsed at a mild severity level (composite score mean = 2.24, standard deviation = 2.51). There was no evidence for sex effects in either of the univariate models, and insomnia symptoms were found to be modestly heritable (similar to 25% at Time 1 and similar to 22% at Time 2). In the longitudinal measurement model, which accounts for error of measurement, the heritability for the latent factor of insomnia symptoms increased substantially, and demonstrated quantitative sex differences. The heritability of the latent insomnia factor was similar to 59% in females and similar to 38% in males. Conclusions: Genetic factors influence insomnia symptoms in adults, moreso for females than males, and these influences are largely stable over time. When taking into account measurement error, heritability estimates are substantial, but unique environmental factors continue to account for a large amount of variance in insomnia symptoms.