The circadian rhythm constitutes an endogenous ~24-hour biological oscillator that enables organisms to entrain physiological processes to periodic environmental cues (e.g., photic and thermal fluctuations), thereby optimizing adaptive behaviors and maintaining metabolic homeostasis—a fundamental biological mechanism critical for health maintenance and disease prevention [1]. This phylogenetically conserved timekeeping mechanism is ubiquitously observed across taxonomic kingdoms, from prokaryotic cyanobacteria to complex mammalian systems [2].

At the molecular level, mammalian circadian regulation operates through a hierarchically organized network of cellular oscillators. The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the central pacemaker, coordinating peripheral clocks through integrated neural and humoral signalling pathways [3, 4]. The core molecular mechanism involves transcription-translation feedback loops (TTFLs), wherein clock gene products autoregulate their expression through sequential post-translational modifications, including phosphorylation and regulated nuclear translocation [4]. Notably, while circadian systems arose through convergent evolution in diverse taxa, they exhibit conserved architectural principles characterized by autoregulatory genetic networks mediated by Per-Arnt-Sim (PAS) domain-containing transcriptional regulators [5]. The groundbreaking work of Nobel laureates Hall, Rosbash, and Young, utilizing Drosophila melanogaster models, identifying period gene-encoded components as essential for establish the autonomous circadian oscillator [6].

Accumulating evidence demonstrates that circadian rhythms represent a fundamental regulatory layer governing virtually all biological processes [7]. Circadian disruption has been etiologically linked to numerous metabolic and endocrine disorders, including obesity, metabolic syndrome, obstructive sleep apnea (OSA), and type 2 diabetes mellitus [8, 9, 10, 11, 12]. This pervasive regulatory influence highlights the circadian system’s integral role in maintaining cellular homeostasis, coordinating physiological functions, and integrating metabolic-endocrine signaling, thereby revealing novel therapeutic targets for associated pathologies [7].

Of particular relevance, circadian oscillations exert precise temporal control over immune system function, modulating host defense mechanisms against inflammatory processes and pathogenic challenges from bacterial, parasitic, and viral agents [13, 14, 15]. Notably, viral replication cycles exhibit distinct temporal regulation patterns, with certain pathogens employing chronobiological strategies to establish either acute or persistent infections [14]. These findings underscore the critical importance of elucidating circadian-immune system interactions in contemporary biomedical research.

This review synthesizes current knowledge regarding circadian regulation of immunological processes. We systematically evaluate: (1) circadian-mediated modulation of host-pathogen interactions; (2) temporal control of leukocyte functional dynamics; (3) circadian light hygiene and gut-immnue crosstalk; and (4) chronobiological regulation of anti-tumor immunity. Furthermore, we examine the clinical implications of circadian biology in immunotherapeutic strategies and chronotherapeutic interventions through a translational medicine perspective. Finally, we identify key research directions to advance understanding of circadian-immune crosstalk, with particular emphasis on mechanistic studies of temporal regulation and its therapeutic applications in precision chrono-immunology.

The mammalian circadian timing system comprises three functionally integrated components: (1) input pathway that transduce environmental signals (e.g., light-dark cycles) to the central pacemaker; (2) a central pacemaker generating self-sustained oscillations; and (3) output pathway that regulate rhythmic physiological, metabolic, and behavioral processes [16]. While the suprachiasmatic nucleus (SCN) of the hypothalamus functions as the dominant circadian pacemaker in mammals, autonomous circadian oscillators exist in peripheral tissues [4]. The SCN synchronizes these peripheral clocks through coordinated neuroendocrine and autonomic nervous signaling [6]. Environmental zeitgebers (“time-givers”), particularly photic stimuli and feeding cycles, entrain the central oscillator, which in turn phase-locks peripheral oscillators via coupled transcriptional-translational feedback loops (TTFLs) [17] (Fig. 1).

Fig. 1.

Molecular architecture and systemic organization of the mammalian circadian clock system. The mammalian circadian timing system operates through a hierarchically organized network, with the central oscillator in the suprachiasmatic nucleus (SCN) serving as the master pacemaker. The SCN integrates photic input and synchronizes peripheral oscillators through dual efferent pathways: (1) autonomic neural signaling and (2) neuroendocrine regulation. At the molecular level, cell-autonomous circadian rhythms are generated by evolutionarily conserved transcriptional-translational feedback loops (TTFLs) comprising core clock components: CLOCK-BMAL1 heterodimers function as primary transcriptional activators by binding to E-box enhancer elements (CACGTG) in promoter regions, thereby initiating the expression of core clock repressors Period [PER1-3] and Cryptochrome [CRY1-2] and numerous clock-controlled genes (CCGs). Accumulated PER-CRY protein complexes form repressive complexes that inhibit CLOCK-BMAL1 transcriptional activity, thereby inhibit their own transcription. Concurrently, CLOCK-BMAL1 activates transcription of nuclear receptors (REV-ERB$\alpha$/$\beta$, ROR$\alpha$/$\beta$/$\gamma$). These regulators competitively bind ROR-responsive elements (ROREs) in the BMAL1 promoter. Illustration was created using Microsoft PowerPoint. CRY, Cryptochrome; PER, Period; ROR, Retinoic acid-related orphan receptor; CLOCK, Circadian locomotor output cycles kaput; BMAL1, Brain and muscle ARNT-like 1.

At the molecular level, cell-autonomous circadian oscillators in both SCN neurons and peripheral cells share a conserved core mechanism centered on interlocking TTFLs [4] (Fig. 1). In the primary transcriptional loop, the core transcriptional activators circadian locomotor output cycles kaput (CLOCK) and brain and muscle ARNT-like 1 (BMAL1) form heterodimers that bind E-box regulatory elements, driving transcriptional activation of numerous clock-controlled genes (CCGs) [11, 18]. Period (PER1-3) and Cryptochrome (CRY1-2) genes, transcripted by CLOCK and BMAL1 heterodimers, accumulate progressively to repress their own transcription, establishing a negative feedback mechanism [19]. Additionally, the CLOCK-BMAL1 complex regulates nuclear receptor superfamily members, including reverse orientation c-erbA gene (REV-ERB$\alpha$/$\beta$, as know as Nr1d1/2) and retinoic acid-related orphan receptor (ROR$\alpha$/$\beta$/$\gamma$) isoforms [18]. These receptors compete for ROR response elements (ROREs) in the BMAL1 promoter: REV-ERBs repress, whereas RORs activate transcription, forming an auxiliary feedback circuit that stabilizes circadian periodicity [18]. Complementing transcriptional regulation, core clock proteins undergo extensive post-translational modifications (e.g., phosphorylation, acetylation, ubiquitination, and SUMOylation), introducing regulatory complexity via modulation of protein stability and subcellular localization [18, 20].

The circadian-immune axis exhibits bidirectional crosstalk that profoundly modulates host defense mechanisms against bacterial, parasitic, and viral infections, with significant implications for inflammatory pathogenesis [21].

The circadian clock system not only orchestrates the body’s defense response to pathogenic invasions but also exerts profound regulatory effects on both innate and adaptive immune responses by temporally controlling immune cell populations and their functional dynamics (Table 1, Ref. [13, 15, 19, 22, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101]). Emerging evidence indicates that circadian rhythmicity governs immune cell trafficking, spatial organization, clonal expansion, lineage commitment, and functional competence through integrated mechanisms involving intrinsic cellular oscillators and extrinsic microenvironmental zeitgebers [21, 58, 59, 102]. At the molecular level, core clock components (e.g., BMAL1 and CLOCK) directly regulate the transcriptional activity of immunologically relevant genes, including those involved in NF-$\kappa$B signaling pathways and pattern recognition receptor expression [103].

Host circadian rhythms are intricately intertwined with the gut microbiome, which exhibits diurnal oscillations in community composition, gene expression, and metabolite production [124, 125]. These microbial rhythms-particularly in the biosynthesis of short—chain fatty acids (SCFAs) like butyrate and acetate—modulate host immune regulation by altering histone acetylation, promoting Treg differentiation, and influencing dendritic cell activation.

Environmental light cues, especially photoperiod length and nocturnal light pollution, are critical for synchronizing both host and microbial circadian clocks. Disruption in light hygiene (e.g., shift work, screen light exposure, irregular sleep) has been shown to reduce microbiota diversity, decrease SCFA levels, and compromise intestinal epithelial integrity [126, 127]. Such disturbances may exacerbate mucosal inflammation and increase susceptigility to infections or autoimmune disorders.

During the COVID-19 pandemic, reduced outdoor light exposure, social isolation, and disrupted sleep cycles led to widespread circadian misalignment. Concomitantly, melatonin suppression, gut dysbiosis, and increased intestinal permeability were observed in numerous patients and convalescents [128]. Emerging evidence further indicates that post-COVID individuals exhibit heightened sensitivity to light cues, highlighting long-term vulnerability of the light–microbiome–immune axis [129].

Notably, melatonin serves as a key mediator in this tripartite relationship. Beyond its role in regulating sleep–wake cycles, melatonin modulates gut immune responses by preserving tight junction protein expression, mitigating ROS-induced damage, and restoring microbial eubiosis [130]. Chronobiological interventions designed to maintain microbial rhythmicity—such as timed light exposure, scheduled meal timing, and melatonin supplementation—hold promise for alleviating post-infectious or stress-induced immune dysregulation.

The circadian clock acts as a pivotal regulator of systemic homeostasis, and its dysregulation promotes tumorigenesis and malignant progression through chronoimmunological perturbations, thereby increasing cancer risk [131, 132]. Murine models exemplify the empirical validation of circadian oncology principles, where the timing of tumor implantation dictates neoplastic growth dynamics, underscoring the temporal dependency of oncological processes [133].

Circadian rhythm disruption (CRD) induces multilevel dysregulation of tumor-associated immune cells while fostering a protumoral and immunosuppressive tumor immune microenvironment (TIME) (Fig. 2). This pathological niche, enriched with dysregulated cytokine/chemokine networks, establishes a self-reinforcing loop that accelerates malignant transformation [134, 135]. CRD-mediated erosion of circadian macrophage polarization rhythms significantly reduces the M1/M2 index (proinflammatory/anti-inflammatory) in splenic and tumoral compartments, creating an immunosuppressive landscape conducive to tumor expansion [136]. Furthermore, CRD disrupts transcriptional-translational coordination of perforin in natural killer (NK) cells, impairing circadian cytotoxic rhythms and accelerating tumor immune evasion [137].

Fig. 2.

Mechanistic impact of Circadian rhythm disruption (CRD) on tumorgenesis. CRD exerts multifaceted immunosuppressive effects that promote tumor progression through the following mechanisms: (1) Disrupting M1/M2 macrophage polarization toward tumor-promoting anti-inflammatory phenotypes; (2) Disrupting circadian-regulated perforin expression in natural killer cells, compromising their tumoricidal capacity; (3) Impairing rhythmic ICAM-1 expression, thereby disrupting leukocyte trafficking; (4) Dysregulating circadian surface expression of CD80 and PD-1, attenuating CD8⁺ T cell activation and potentiating immune checkpoint pathways; (5) Inducing tumor-specific CXCL5 overexpression that facilitates CXCR2⁺ myeloid-derived suppressor cell recruitment, establishing an immunosuppressive niche; (6) Directly inhibiting cytotoxic T lymphocyte infiltration. These coordinated perturbations in chronoimmunological regulation establish a permissive microenvironment for multifactorial immunosuppression and malignant progression. Red vertical upward arrows indicate cell activation or gene upregulation; red vertical downward arrows indicate cell activity inhibition or gene downregulation; brown arrows indicate promotion. Diagram was assembled in Microsoft PowerPoint.

Emerging evidence highlights TIME modulation and immune infiltration dynamics as circadian-regulated hallmarks of cancer. Endothelial cells exhibit circadian ICAM-1 expression patterns that govern leukocyte trafficking rhythms into tumors [138]. CD8⁺ T cells demonstrate time-of-day-dependent functional oscillations, transitioning from immunosuppressed morning states to enhanced anti-tumor activity in the evening [138]. BMAL1-mediated circadian control of CD80 in dendritic cells coordinates rhythmic CD8⁺ T cell priming in tumor-draining lymph nodes, establishing temporal windows for optimal anti-tumor immunity [133]. CRD-induced cytokine network perturbations disrupt leukocyte circadian trafficking, reducing circulatory immune cell numbers and tumor infiltration capacity [139]. Mechanistically, CRD-driven TIME reprogramming involves activation of CXCL5-CXCR2 axis, which recruits immunosuppressive myeloid cells that suppress T cell responses and block CD8⁺ T cell infiltration [139]. These findings establish the clinical relevance of clock genes and their transcriptional targets (e.g., CXCL5, CD80) as prognostic chronotherapeutic biomarkers in oncology [140, 141, 142, 143].

Collectively, circadian regulatory networks represent a fundamental axis in cancer biology. Deciphering circadian-cancer crosstalk may unveil chronotherapeutic strategies for precision oncology, leveraging temporal biology to optimize immune activation and tumor suppression.

The circadian clock acts as a central regulator of immune homeostasis, exerting both direct cellular and systemic modulatory effects. Beyond the established crosstalk between core clock components and immune cells, emerging evidence highlights circadian-mediated immunomodulation through neuroendocrine, metabolic, and hypoxic signaling axes. For example, glucocorticoids (GCs) rhythmically control T-cell trafficking via GR-mediated binding to glucocorticoid response elements (GREs) in the IL-7R$\alpha$ locus enhancer, thereby regulating CXCR4 expression cycles [116, 144]. A bidirectional regulatory loop exists between hypoxia-inducible factor-1$\alpha$ (HIF-1$\alpha$) and core circadian machinery [145], with experimental models showing that HIF-1$\alpha$ forms a transcriptional complex with ARNTL1/BMAL1 and CLOCK. This complex binds E-box elements in the CXCR4 promoter to orchestrate leukocyte migration, a process modulated by circadian-controlled ROS levels and p38MAPK/MK2 phosphorylation cascades that stabilize HIF-1$\alpha$ [146, 147, 148]. Additionally, circadian-driven sympathetic activity releases phase-dependent norepinephrine, which activates $\beta$2-adrenergic receptors ($\beta$₂-ARs) on lymph node lymphocytes [105, 149]. This neuroimmune interaction upregulates CCR7/CXCR4 chemokine receptors through cAMP-PKA signaling, retaining lymphocytes within lymphoid compartments [105, 115, 117].

Chronoimmunological research has uncovered significant temporal variations in immune regulation with direct clinical relevance. The CXCL12-CXCR4 axis has emerged as a therapeutic target in strategies combining CXCR4 inhibitors with immunotherapy to overcome treatment resistance [139]. Circadian synchronization studies reveal robust oscillatory expression patterns of the Pdcd1 gene (encoding PD-1) in activated CD8⁺ T cells, with corresponding diurnal variations in the efficacy of anti-PD-1 and CAR T-cell therapies [138]. Vaccination timing has been shown to influence tumor growth modulation in preclinical models [133, 138], while circadian dysregulation is implicated in severe COVID-19 pathogenesis [150]. Melatonin administration holds therapeutic potential for mitigating cytokine storms in critical cases [151], and nocturnal bright light therapy—by suppressing plasma melatonin—has been proposed as adjuvant for malaria eradication [152]. These findings underscore the transformative potential of temporally optimized therapies for immune-related disorders.

Substantial mysteries remain regarding the bidirectional circadian-immune system regulation. While core clock components like BMAL1 are established modulators of immune cell rhythms, the role of auxiliary clock proteins in functional attenuation require systematic investigation. Methodologically rigorous approaches—such as cell-type-specific circadian disruption models—are essential to map clock protein domains to immunoregulatory phenotypes. Single-cell transcriptomics should be prioritized to dissect cell-type-specific circadian-immune crosstalk, including how tissue-resident macrophages maintain local rhythms independent of SCN signals control. Development of small-molecule chronotherapeutics (e.g., BMAL1 stabilizers or CRY degraders) may offer precision strategies for autoimmune diseases. Elucidating these mechanisms could advance our understanding of disease pathogenesis and identify novel therapeutic targets.

Recent studies highlight the importance of nocturnal pineal melatonin and cortisol profiles in resetting immune cells for diurnal function, with implications for cancer and neurodegenerative conditions [153]. Melatonin peaks during early sleep, followed by a cortisol rise during the cortisol awakening response. Like gut-derived butyrate, melatonin inhibits nuclear translocation of glucocorticoid receptor-alpha (GR-$\alpha$) [153, 154], indicating that factors suppressing pineal melatonin (e.g., aging, type 2 diabetes) [128] may disinhibit cortisol-mediated immune regulation. Clarifying how circadian alterations impact immune cell gene expression and mitochondrial function represents a key future direction.

Understanding how environmental zeitgebers (e.g., light exposure, gut microbial rhythms) integrate into circadian immune networks—as discussed in Section 5—will be essential for developing holistic chrono-immunological interventions. Such insights may bridge basic chronobiology with translational medicine, fostering temporally optimized strategies to enhance immune health.

YJ, CL, and YZ contributed to the conceptualization and writing of the manuscript. YJ and YZ assembled the figures and tables. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

Not applicable.

This work was supported by grants from the National Key R&D Program of China (2019YFA0800400), the National Natural Science Foundation of China (NSFC) (31871186), the Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment (CURE), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions (YX13400214).

The authors declare no conflict of interest.

During the preparation of this work the authors used DeepSeek in order to check spell and grammar. After using this tool, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication.