Sleepless nights may be genetically linked to depression, according to new research from the University of Pennsylvania and Virginia Commonwealth University. In a study of twins, researchers found that genetically identical twins who suffered from insomnia were significantly more likely than nonidentical twins to also suffer from depression. The two disorders have been linked before, but the role of genetics has not been clear. The new study indicates that insomnia and depression have overlapping genes, and the next step is to pinpoint those genes through DNA analysis. Possible contenders are the genes related to the neurotransmitters serotonin and norepinephrine, which are involved in both the sleep-wake cycle and mood regulation.