This chapter reviews the sleep and circadian rhythms disturbances observed in psychiatric disorders, as well as their possible associations with circadian clock genes. Recent studies have provided interesting data on associations between clock gene polymorphisms and major depressive disorders (MDD). Seasonal affective disorder (SAD) is another common mood disorder, affecting around 10% of the population living in temperate latitudes. Bipolar disorder (BPD) is a complex disorder arising from the inheritance of multiple genetic variants in which patients alternate between episodes of mania and depression. Schizophrenia affects about 1% of the general population. It is a complex and severe psychiatric disorder characterized by profound disruptions of cognition, emotion and social functions. In the last decade, there has been mounting evidence to suggest that circadian rhythm deficits play a key role in most psychiatric disorders.