Abstract
• Impairments of the circadian system are associated with mood disorders. • Clock genes, neurotransmitter cycles, and circadian rhythms may serve as biomarkers. • Current antidepressants function as exogenous modulators of the circadian system. • Chronobiological therapies represent an effective addition to conventional treatments. The prevalence of mood disorders in the population is the highest among the diagnosed mental illnesses. In the U.S., an estimated 21.4% of adults have experienced a clinically significant mood disorder at some time in their lives. Mood disorders are characterized by dysfunctional affective responses associated with emotional regulation alterations, thoughts' contents, and volition. Among the symptoms of mood disorders, disturbances of sleep and activity patterns are recognized as core diagnostic criteria. Despite the high degree of comorbidity between dysfunctional mood and sleep/activity patterns, the biological underpinnings common to the two illnesses are still unknown. The alternation of sleep and wakefulness follows a circadian cycle, determined by the synchronization between the external environment and the organism's internal clock. Virtually, the body's circadian system influences all aspects of an organism's functioning, including gene regulation, cellular communication, cognitive performance, and emotional states. In this context, the association between mood disorders and the circadian system appears critical in the etiology of this class of mental diseases. This review aims to integrate the vast heterogeneity of findings linking mood with circadian factors. The picture that emerges indicates that mood is strongly influenced by the interaction with different components of the circadian system, such as exogenous modulators of the circadian system (e.g., food, drugs, and light exposure), clock gene alterations, and behavioral patterns. Advances in comprehending the complex relationship between mood and the biological cycles would lead to identifying novel biomarkers of mood disorders, with future improvements in treatment optimization.
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