Growing evidence indicates that the gut microbiota is not a static ecosystem but a rhythmic metabolic organ whose oscillatory activity is tightly coordinated with host circadian biology. Disruption of this temporal alignment, through irregular diet, sleep disturbance, shift work, or social jet lag, may profoundly alter microbial composition and the production of neuroactive metabolites. These alterations have emerged as potential contributors to the pathophysiology of mood disorders. This review introduces the concept of chrono-metabolic psychiatry, a framework integrating circadian rhythms, gut microbiota dynamics, and host metabolic signaling in the development and course of depressive and bipolar disorders. In this framework, the term "chrono-metabolic" refers to the integration of biological timing, host metabolic regulation, and microbiota-derived metabolic signaling. Chrono-metabolic psychiatry therefore shifts the focus from static dysbiosis or neurotransmitter imbalance alone to the time-dependent interactions among circadian misalignment, microbial rhythmicity, immune regulation, metabolite production, and affective instability. Diurnal fluctuations in short-chain fatty acids, tryptophan-kynurenine metabolites, bile acids, and microbial-derived neurotransmitters interact with clock gene regulation, hypothalamic-pituitary-adrenal axis activity, neuroinflammation, and synaptic plasticity. Chrono-disruption may represent a transdiagnostic vulnerability factor and may confirm the bidirectional relationship between mood instability and microbiota rhythmicity. Emerging therapeutic implications, including chrono-nutrition, time-restricted feeding, targeted probiotic administration ("chronobiotics"), and the microbiota-modulating effects of psychotropic medications are discussed. By shifting from a compositional to a temporal-metabolic perspective, this model highlights the importance of microbial oscillations rather than static dysbiosis alone. Integrating circadian biology into microbiota research may enable metabolomic stratification and pave the way for precision psychiatry approaches grounded in host-microbe metabolic crosstalk. Future longitudinal and time-resolved multi-omics studies are needed to validate this framework and to translate it into clinically actionable interventions.
