Insomnia is a well-recognised risk for depression but it is not well known if depression induces insomnia in turn, or if there were genetic factors in common for both features. We investigated the direction of causality between problems in sleep and depressed mood, and also whether genes in common underlie both traits, in a nation-wide cohort sample of 18 631 same-sex twins with a follow-up interval of six years. As an approximation for depressed mood, we used information on life dissatisfaction that has previously been found to be strongly associated to symptoms of depression and with high specificity and sensitivity to the disorder itself (Paunio et al. submitted). Most individuals (59 %) with new-onset life dissatisfaction had experienced suboptimal sleep at the baseline. Poor sleep predicted life dissatisfaction in a consistent pattern (OR=2.1 from logistic and 3.0 from conditional logistic regression analysis), but life dissatisfaction did not predict poor sleep. There was substantially heritability for both traits, but their shared genetic component was relatively weak (genetic correlation of .21 for males and .27 for females in a multivariate genetic model). These results suggest that most of the association between symptoms of insomnia and depressive disorder is due to an effect of poor sleep on mood. Majority of the contributing genetic factors were unique, consistent with the hypothesis that poor sleep has a neurophysiological effect on brain, emotions and mood. Finally, revelation of genetic factors contributing to experiences of poor sleep will bring new insight into genetic background of depressive disorder as well.

Paunio, T., Korhonen, T., Hublin, C., Partinen, M., Kivimäki, M, Koskenvuo, M, Kaprio, J. Manuscript submitted