Sleep disturbances in major depressive disorder (MDD) are characterized by increased sleep latency, poorer sleep efficiency reduced latency to the first rapid eye movement (REM) sleep episode, and early-morning awakening, but there is little data to indicate a role of circadian clock genes in MDD.
Furthermore, a study examining the association between Clock gene polymorphisms and insomnia revealed a higher recurrence of initial, middle, and terminal insomnia in patients homozygous for the Clock genotype.