Aims: To study the impact of biological rhythm disturbance in patients with bipolar depression. Materials and Methods: Case-control study was carried at Jawaharlal Nehru Medical College, Aligarh Muslim University, U.P, India. 100 cases of bipolar depression and 100 healthy controls were recruited. Biological Rhythms Interview of Assessment in Neuropsychiatry Questionnaire (BRIAN) and Functioning Assessment Short Test (FAST) were used to assess biological rhythms and functioning, respectively. HAM-D scale was used to assess severity of depressive symptoms. Results: Total as well as certain domain-specific BRIAN, FAST & HAM-D scores were significantly higher in cases compared to controls. Conclusions: biological rhythm dysfunction has been found to be common in bipolar depression.