Abstract
The ~12-h ultradian rhythm (circasemidian) represents an evolutionarily conserved temporal architecture that complements the canonical 24-h circadian clock. Over the past 5 years, mounting evidence has revealed its ubiquity across biological kingdoms, from tidal marine organisms and cyanobacteria to plants, microbiomes, and mammals, including humans, manifesting as intrinsic oscillations in gene expression, metabolism, and behavior that often persist independently of circadian control. In mammals, this rhythm is driven by a cell-autonomous oscillator centered on the XBP1s (X-box binding protein 1)/IRE1α (Inositol requiring enzyme 1 alpha) axis, orchestrating endoplasmic reticulum stress responses and lipid homeostasis through negative feedback regulation, further reinforced by metabolic coupling and bidirectional crosstalk with circadian pathways. Functionally, 12-h oscillations act as a secondary temporal layer that ensures bimodal photostatic and energetic homeostasis, synchronizing multi-organ physiology across the day-night transition. Pathologically, disruption of this rhythm contributes to metabolic syndromes (e.g., NAFLD (non-alcoholic fatty liver disease), diabetes), neuropsychiatric disorders (e.g., schizophrenia), and age-related dysfunctions, particularly within ocular and immune systems. Despite accumulating correlative and model-based evidence, causal mechanisms remain insufficiently defined, and human data are limited. Future work integrating multi-omics chrono profiling, comparative genomics, and clinical chronotherapeutic trials will be critical to delineate this semidiurnal oscillator’s molecular architecture and translational potential, thereby advancing precision chrono medicine and expanding the current paradigm of biological timing.