A longitudinal twin and sibling study of associations between insomnia and depression symptoms in young adults

Study objectives: To estimate genetic and environmental influences on the associations between insomnia and depression symptoms concurrently and longitudinally.
Design: Longitudinal twin/ sibling study.
Setting: Population based twin/ sibling registry from the U.K.
Patients or Participants: One thousand five hundred and fifty six twins and siblings participated at Time 1 (mean age = 20.3 years, SD = 1.76). Eight hundred and sixty two participated at Time 2 (mean age = 25.2 years, SD = 1.73 years).
Interventions: N/A
Measurements and Results: The Insomnia Symptoms Questionnaire and the Short Mood and Feelings Questionnaire were used to assess symptoms of insomnia and depression respectively. Genetic effects accounted for 33-41% of the variance of the phenotypes. The phenotypic correlations were moderate (r = .34 to r = .52). The genetic correlations between the variables were high (.73 – 1.00). Genetic effects accounted for a substantial proportion of the associations between variables (50-90%). Non-shared environmental effects explained the rest of the variance and covariance of the traits.
Conclusions: While genetic effects play a modest role in insomnia and depression symptoms separately, they appear to play a more central role in concurrent and longitudinal associations between these phenotypes. This should be acknowledged in theories explaining these common associations.