Living organizms show cyclic rhythmicity in a variety of physiological, hormonal, behavioral, and psychological processes. Sleep-wake cycles, body temperature, hormone levels, mood and cognition display a circadian rhythm in humans. Delays, advances or desynchronizations of circadian rhythm are known to be strongly associated with mental illness especially mood disorders such as bipolar disorder, major depression and seasonal affective disorder. Furthermore, some of the mood stabilizers, sleep deprivation and light treatment are employed to treat mood disorders by shifting circadian rhythm. This paper reviews the relationship between mood disorders and circadian rhythm, and describes treatment options by altering circadian rhythm.