CLOCK and depressive disorder: In mood disorders such as depression, the sleep–wake rhythm is often disrupted, which sometimes leads to circadian rhythm sleep–wake disorders.4,5 The recurrence of mood episodes in bipolar disorder is also associated with circadian rhythm disturbances.6,7 By affecting the circadian rhythms, light therapy can alleviate mood symptoms.8 Basic research has indicated a link between circadian rhythm mechanisms, including clock genes, and mood regulation.9,10 Thus, depression and other mood disorders are strongly related to circadian rhythms.