Circadian rhythm disruption and its role in bipolar depression: A neurobiological review

Bipolar disorder is a chronic and disabling psychiatric condition characterized by alternating episodes of mania/hypomania and depression. Among the myriad of biological disturbances implicated in its pathophysiology, circadian rhythm dysregulation has emerged as a significant contributor, particularly in bipolar depression. Circadian rhythms, orchestrated by the suprachiasmatic nucleus (SCN) in the hypothalamus, govern sleep-wake cycles, hormonal release, body temperature, and other physiological functions. Disruptions in these rhythms have been consistently linked to mood instability, sleep disturbances, and increased relapse risk in bipolar individuals. This review explores the intricate neurobiological relationship between circadian dysregulation and bipolar depression, focusing on genetic, molecular, and neuroendocrine pathways. Evidence from clinical and preclinical studies highlights abnormalities in melatonin secretion, CLOCK gene polymorphisms, and altered expression of circadian proteins in patients with bipolar disorder. Furthermore, the bidirectional influence of circadian misalignment and mood dysregulation is considered, demonstrating how lifestyle factors like shift work, light exposure, and irregular sleep patterns exacerbate symptom severity. Chronotherapeutic interventions such as light therapy, interpersonal and social rhythm therapy (IPSRT), and pharmacological agents like lithium and agomelatine provide promising avenues to restore circadian balance and improve clinical outcomes. This review underscores the importance of integrating circadian-focused strategies into the treatment and management of bipolar depression and calls for further translational research to elucidate causal mechanisms and optimize therapeutic targets.