Lewis KJS, Richards A, Karlsson R, et al. - Researchers examined whether polygenic risk scores for sleep traits could differentiate subtypes of bipolar disorder. They conducted a case-control study in the United Kingdom and Sweden including 4,672 participants with bipolar disorder and 5,714 control participants. In this work, for the first time, associations between polygenic liability to insomnia and hypersomnia and clinical strata within bipolar disorder are conferred. The analysis revealed significantly greater genetic liability to longer sleep duration among individuals with bipolar disorder I, whereas significantly greater genetic liability to insomnia was noted among individuals with bipolar disorder II. Replication of these findings was done in an independent sample. Genetic liability to morning or evening chronotype did not differ among individuals with bipolar subtypes.