A Longitudinal Twin and Sibling Study of Associations between Insomnia and Depression Symptoms in Young Adults

To estimate genetic and environmental influences on the associations between insomnia and depression symptoms concurrently and longitudinally.

Behavioral genetic analyses were conducted on data from the British longitudinal G1219 twin/sibling study. One thousand five hundred fiftysix twins and siblings participated at Time 1 (mean age = 20.3 years, SD = 1.76). Eight hundred sixty-two participated at Time 2 (mean age = 25.2 years, SD = 1.73 years). Participants completed the Insomnia Symptoms Questionnaire and the Short Mood and Feelings Questionnaire to assess symptoms of insomnia and depression respectively.

Genetic effects accounted for 33% to 41% of the variance of the phenotypes. The phenotypic correlations were moderate (r = 0.34 to r = 0.52). The genetic correlations between the variables were high (0.73-1.00). Genetic effects accounted for a substantial proportion of the associations between variables (50% to 90%). Non-shared environmental effects explained the rest of the variance and covariance of the traits.

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.