Disturbances in the circadian rhythms have long been associated with depression and mania. Animal models of mania and depression exhibit differential effects upon the intrinsic circadian period and the same occurs with antidepressants and mood stabilizers treatment. The intrinsic circadian period is expressed when there are no time clues or when the light/dark cycle length is beyond the capacity of synchronization. In summary, while there is no clear association between the circadian period and mania, depressive-like behaviour is generally associated either with lengthening of the circadian period or with arrythmicity, and the improvement of depressive-like behaviour is associated with shortening of the circadian period. Thus, this review is an attempt to summarize data regarding these correlations and find a putative role of the circadian intrinsic period in mood regulation, particularly concerning the switch from depression to mania.