Two new studies draw correlations between shortened sleep and depression, reports The Philadelphia Inquirer.

In the first study, published in the journal Sleep, researchers at the University of Washington in Seattle found a link between the heritability of depressive symptoms and sleep duration in 1,788 twins. Participants who slept less than seven hours a night “magnified the genetic influence toward depressive symptoms,” as did, to a lesser extent, twins who slept more than nine hours a night. For the study, the optimal amount of sleep was considered to be between seven and nine hours.