INTRODUCTION
Circadian rhythms are self-sustaining biological processes that oscillate on an approximately 24-hour cycle and regulate physiology and behavior including sleep, metabolism, and hormone secretion [1]. These rhythms are controlled by two interconnected systems: the central clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus and peripheral clocks found in most tissues throughout the body [2]. The SCN’s principal synchronizer—known as a “zeitgeber” (German for “time giver”)—is light detected through the retina, which aligns internal rhythms with the external day–night cycle. The SCN subsequently synchronizes peripheral clocks through neurological and hormonal signals, including autonomic nervous system activity and glucocorticoid release [3]. Additional zeitgebers include meal timing, physical activity, and temperature. Alignment between these clocks appears to be important for metabolic health; misalignment—as occurs in shift work, jet lag, and irregular sleep schedules—compromises cardiometabolic function and increases disease risk [4,5].
Molecular architecture of the circadian clock
At the molecular level, circadian rhythms arise from interlocking transcription-translation feedback loops (TTFLs). In the primary loop, the transcription factors BMAL1 (brain and muscle ARNT-like 1) and CLOCK (circadian locomotor output cycles kaput) dimerize, bind to E-box enhancer elements, and activate transcription of Period (Per1, Per2, Per3) and Cryptochrome (Cry1, Cry2) genes [6]. Following translation, PER and CRY proteins form complexes that translocate to the nucleus and directly inhibit BMAL1:CLOCK activity, thereby repressing their own transcription. Progressive degradation of PER and CRY proteins—mediated by casein kinase 1δ/ε and ubiquitin-proteasome pathways—releases this inhibition and permits reactivation of BMAL1:CLOCK, completing the cycle approximately every 24 hours [7].
A secondary auxiliary loop adds regulatory complexity and stability. The nuclear receptors REV-ERBα/β (encoded by *Nr1d1/2*) and RORα/γ (retinoid-related orphan receptors) compete for ROR response elements (ROREs) in the Bmal1 promoter: REV-ERBs repress Bmal1 transcription, while RORs activate it [8]. This interplay generates robust, high-amplitude oscillations in Bmal1 expression and contributes to clock resilience against environmental and metabolic perturbations.
Circadian regulation of metabolism
Clock genes regulate an estimated 40% of protein-coding genes in a tissue-specific manner, including numerous metabolic genes [9]. A canonical example is Nampt (nicotinamide phosphoribosyltransferase), the rate-limiting enzyme in the NAD+ salvage pathway. Nampt expression is directly controlled by BMAL1:CLOCK, generating circadian oscillations in intracellular NAD+ levels [10]. Clock-mutant mice lack these NAD+ oscillations, demonstrating the clock’s role in metabolic anticipation [11].
The circadian regulation of metabolism can be conceptualized as a temporal transition between anabolic and catabolic states. During the day, which represents the anabolic phase in diurnal humans, high energy availability (high ATP, low AMP) suppresses AMPK (AMP-activated protein kinase) and SIRT1 (sirtuin 1) enzymatic activity, thereby permitting anabolic processes including glycolysis, lipogenesis, and mTOR activation. Concurrently, clock-driven transcription increases expression of metabolic enzymes in anticipation of future demands [12]. In contrast, during the night (catabolic phase), low energy availability (high AMP/ADP, elevated NAD+) activates AMPK and SIRT1. AMP allosterically activates AMPK, which is phosphorylated by LKB1 (liver kinase B1). AMPK activation stimulates NAMPT, increasing NAD+ and activating SIRT1. Active SIRT1 deacetylates and mobilizes LKB1, creating a positive feedback loop that promotes catabolic processes, including fatty acid oxidation, mitochondrial biogenesis, and autophagy [13].
This tight integration between circadian and metabolic pathways suggests that the clock may be a potentially important therapeutic target for metabolic diseases.
METHODS
This narrative mini-review was based on a targeted literature search primarily conducted in PubMed and related databases, focusing on studies of circadian regulation, NAD+/SIRT1 signaling, ROR-mediated clock modulation, and key preclinical and human studies involving NMN, resveratrol, and nobiletin. References were selected to provide a conceptual framework for evaluating candidate circadian amplifiers, with emphasis on seminal mechanistic reports, representative animal studies, and currently available human investigations. This review was not designed as a formal systematic review. Therefore, the search and selection process was intended to provide a focused narrative overview rather than an exhaustive evidence synthesis.
DEFINING “CIRCADIAN AMPLIFICATION”
Before evaluating candidate compounds, it is essential to operationally define what constitutes a “circadian amplifier.” The term has been used variably in the literature, leading to potential confusion. For this review, we adopt the following framework based on prior proposals [14,15].
Operational definitions
For the purpose of this review, circadian amplification refers to interventions hypothesized to increase the robustness, precision, and/or amplitude of circadian oscillations without necessarily altering period length or phase. Key dimensions include: 1) molecular amplitude—increased peak-to-trough ratio of core clock gene expression (e.g., Per2, Bmal1) or reporter activity (e.g., PER2::LUC bioluminescence) in cells or tissues; 2) cellular synchrony—enhanced phase coherence among individual oscillators within a tissue, reducing desynchrony; 3) physiological robustness—improved stability of behavioral rhythms (rest-activity, body temperature) under constant conditions or after perturbation; and 4) resilience—accelerated re-entrainment following phase shifts (jet lag) or reduced rhythm fragmentation with aging.
Preclinical readouts
Common experimental approaches for assessing circadian amplification include in vitro PER2::LUC bioluminescence recording in fibroblasts or tissue explants to measure rhythm amplitude, damping rate, and synchrony [16]; in vivo assessment of voluntary wheel-running activity or actigraphy in rodents to evaluate period, amplitude, and stability under light–dark cycles, constant darkness, and following phase shifts) [17]; and molecular analyses, including quantitative PCR, RNA sequencing of clock gene expression across multiple time points, and chromatin immunoprecipitation for BMAL1/CLOCK binding.
Translation to human endpoints
For human studies, circadian amplification can be assessed through behavioral rhythms, circadian phase markers, molecular markers, and metabolic outcomes. Behavioral rhythms can be evaluated using actigraphy-derived metrics, including interdaily stability, intradaily variability, and relative amplitude [18]. Circadian phase markers include dim-light melatonin onset (DLMO), cortisol acrophase, and core body temperature minimum. Molecular markers may be accessed through circadian gene expression in peripheral blood mononuclear cells or oral mucosa across multiple time points [19]. In addition, metabolic outcomes such as oral glucose tolerance, insulin sensitivity, and energy expenditure may be evaluated, with attention to time-of-day effects. This operational framework is used here to evaluate the three candidate compounds.
CANDIDATE CIRCADIAN AMPLIFIERS
Nicotinamide mononucleotide
Proposed mechanism
Nicotinamide mononucleotide (NMN) is a direct precursor of nicotinamide adenine dinucleotide (NAD+). Oral or intraperitoneal NMN administration elevates NAD+ levels in multiple tissues [20]. Increased NAD+ activates SIRT1, an NAD+-dependent deacetylase that modulates circadian clock function through complementary effects on BMAL1 and PER2 [21]. Acetylation inhibits BMAL1 transcriptional activity, whereas SIRT1-mediated deacetylation at Lys537 enhances BMAL1 binding to target gene promoters, potentiating the activating arm of the TTFL [22]. In contrast, acetylation stabilizes PER2 protein, while deacetylation by SIRT1 promotes PER2 degradation and attenuates the repressive arm of the TTFL [23]. This bimodal regulation is hypothesized to increase the amplitude and precision of circadian oscillations, conceptualized as an “enzymatic rheostat” linking cellular energy status to clock function [24].
Preclinical evidence
In mice, NMN supplementation (500 mg/kg/day intraperitoneally for 6–12 months) produced multiple effects consistent with circadian amplification and metabolic improvement [25], including attenuated age-related weight gain, increased energy expenditure and physical activity, improved insulin sensitivity (homeostatic model assessment of insulin resistance, HOMA-IR), enhanced glucose tolerance, favorable lipid profile modifications, and ameliorated retinal function. Although direct circadian measurements remain limited, these metabolic benefits may plausibly involve circadian mechanisms.
Human evidence
A 12-week randomized, double-blind, placebo-controlled trial investigated NMN (250 mg/day orally) in 108 older Japanese adults (mean age 65–75 years) [26]. The study reported improvements in sleep quality, as assessed by the Pittsburgh Sleep Quality Index, and reductions in daytime sleepiness. Secondary analyses suggested improvements in physical performance, including gait speed and grip strength in selected subgroups. No serious adverse events were reported. This single trial provides preliminary evidence of tolerability and possible benefit for sleep-related outcomes. However, several limitations warrant caution. Circadian outcomes were not directly measured, as no measures of DLMO, actigraphy, or clock gene expression were included. In addition, dosing timing was not optimized or reported (participants instructed to take capsules “at breakfast”). The reported effect sizes were modest, subgroup analyses were underpowered, and the 12-week duration was insufficient to assess long-term safety. Furthermore, the generalizability was limited to healthy older Japanese adults. Thus, while NMN is of interest as a clock-modulating candidate, direct evidence for circadian amplification in humans is currently lacking.
Resveratrol
Proposed mechanism
Resveratrol (3,5,4’-trihydroxy-trans-stilbene) is a natural polyphenol found in grapes, red wine, and various plants. It gained prominence as a putative SIRT1 activator, although its mechanisms are more complex and debated [27]. With respect to circadian regulation, resveratrol allosterically activates SIRT1, thereby increasing its affinity for acetylated substrates, including BMAL1 and PER2 [28]. In addition, AMPK activation (via SIRT1 or upstream kinases) and PGC-1α modulation may contribute to metabolic effects [29]. Through these SIRT1-related pathways, resveratrol may theoretically influence circadian amplitude via the same BMAL1/PER2 deacetylation mechanisms as NMN.
Distinguishing mechanisms from demonstrated clock effects
A critical distinction should be made between proposed mechanisms and demonstrated circadian effects. While resveratrol activates SIRT1 in cell-free assays and some cellular contexts, its effects in vivo are complicated by several factors. Poor bioavailability resulting from rapid glucuronidation and sulfation limits plasma concentrations [30]. In addition, resveratrol exhibits pleiotropic actions on numerous molecular targets, including COX-1/2, estrogen receptors, and sirtuins other than SIRT1 [31]. Furthermore, large clinical trials for metabolic and cardiovascular endpoints have yielded mixed results, indicating inconsistent human efficacy [32].
Direct in vivo evidence for circadian clock modulation remains limited. In mice, resveratrol (200 mg/kg/day in diet) attenuated high-fat diet-induced disruption of clock gene expression in liver and adipose tissue [33]. However, whether this represents direct clock amplification versus secondary effects of improved metabolic health is unclear.
Nobiletin
Mechanism and evidence grading
Nobiletin (5,6,7,8,3’,4’-hexamethoxyflavone) is a polymethoxylated flavone derived from citrus peels. Unlike NMN and resveratrol, nobiletin targets the auxiliary clock loop directly [34]. Nobiletin competitively binds to the ligand-binding domains of RORα and RORγ with higher affinity for RORγ. In cell-based reporter assays, nobiletin activates ROR transcriptional activity [35]. In fibroblasts, nobiletin increases Bmal1 expression and PER2::LUC bioluminescence amplitude, indicating enhanced molecular oscillation [34].
Evidence from animal studies further supports its circadian regulatory effects. In diet-induced obese (DIO) mice, nobiletin (200 mg/kg/day orally) increased locomotor activity and energy expenditure in a clock-dependent manner. Effects were abolished in Bmal1 knockout mice, confirming clock gene dependence [34]. Nobiletin-treated DIO mice showed reduced weight gain, improved glucose tolerance, and favorable metabolic profiles compared to untreated DIO controls [34]. To date, however, no published human trials of nobiletin for circadian or metabolic outcomes.
Critical assessment
Among the three compounds, nobiletin currently provides the most direct preclinical evidence consistent with circadian amplification, with clear demonstration of ROR-mediated clock gene modulation and clock-dependent metabolic effects in animals. However, human translational evidence is currently lacking.
Table 1 shows a structured comparison of the three candidate circadian amplifiers across key evidence domains.
CHRONOPHARMACOLOGY: TIMING CONSIDERATIONS
The three pillars of chronotherapy have been articulated as: 1) training the clock (lifestyle interventions); 2) clocking the drug (timing administration to disease rhythms); and 3) drugging the clock (direct clock modulation) [15]. For these candidate compounds, dosing time may be biologically relevant, but this question remains largely untested empirically.
For NMN, NAD+ levels exhibit circadian oscillations, peaking during the active phase (daytime in humans) [36]. Administering NMN in the morning might synergize with endogenous NAD+ rhythms, though this remains speculative. For resveratrol, given its poor bioavailability, co-administration with meals (particularly high-fat meals) may enhance absorption [37]. However, timing relative to clock function has not been studied. For nobiletin, ROR nuclear receptor activity shows circadian variation. If nobiletin acts as an ROR agonist, administration during the natural peak of ROR transcriptional activity might maximize effects—but this peak timing is tissue-specific and not established for human dosing. To date, no studies have systematically compared morning versus evening dosing with respect to circadian outcomes for any of these compounds. This represents a critical knowledge gap.
TRANSLATIONAL CHALLENGES AND FUTURE DIRECTIONS
Regulatory and quality considerations
Because NMN, resveratrol, and nobiletin are marketed as dietary supplements in many jurisdictions, issues of standardization, formulation, and regulatory interpretation should be considered when discussing translational potential. Several translational challenges remain. Supplement quality, purity, and bioavailability vary substantially among manufacturers, and standardized formulations are lacking [38]. In addition, the optimal human doses for circadian effects are unknown. Animal-to-human dose translation is imprecise. Long-term safety data for these compounds are also limited, particularly for nobiletin. Furthermore, potential interactions with medications (e.g., anticoagulants for resveratrol) require further investigation.
Priority research directions
Future studies must incorporate direct circadian readouts (DLMO, actigraphy, peripheral clock gene expression) rather than inferring circadian effects from metabolic improvements. Randomized comparisons of morning versus evening administration are essential for rational chronotherapy. In addition, stratified analyses by chronotype, age, shift work status, and metabolic health may identify populations most likely to benefit from these interventions. Testing these supplements in combination with lifestyle interventions (time-restricted eating, morning light exposure) may reveal synergistic effects. Finally, mechanistic human studies incorporating tissue-specific clock measurements (adipose, muscle biopsies) across multiple time points would provide direct evidence of clock modulation.
CONCLUSION
Circadian rhythm disruption is increasingly recognized as an important contributor to metabolic disorders including obesity and type 2 diabetes. Strengthening the body’s internal clock, discussed here under the operational concept of circadian amplification, represents a conceptually appealing therapeutic direction. NMN, resveratrol, and nobiletin have emerged as candidate compounds with potential circadian-amplifying properties with distinct molecular mechanisms: NMN and resveratrol target the NAD+/SIRT1 axis to modulate core TTFL components, while nobiletin directly activates ROR nuclear receptors in the auxiliary loop.
Preclinical evidence supports further investigation of the circadian-amplifier concept, particularly in the case of nobiletin, which demonstrates clock gene-dependent metabolic effects in animals. However, human evidence remains nascent. One NMN trial suggests improvements in sleep quality, but circadian-specific outcomes were not measured. Resveratrol has extensive human metabolic data but no circadian studies. Nobiletin lacks human data entirely.
Accordingly, any claim that these compounds directly amplify circadian rhythms in humans should be considered preliminary. The field must advance from plausible mechanisms to rigorous clinical investigation incorporating direct circadian readouts, optimized timing strategies, and careful responder identification. With stronger mechanistic and clinical evidence, circadian amplification may eventually emerge as a useful component of chronotherapeutic approaches to metabolic health.
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