Short-term continuous light exposure induces hippocampal rhythmic and functional alterations: a multi-timepoint metabolomics study

Circadian rhythms constitute an intrinsic timekeeping system that allows organisms to adapt to the 24-h day–night cycle, coordinating physiological activities, behavioral patterns, and metabolic processes to sustain the orderly progression of biological functions (Allada and Bass, 2021). The suprachiasmatic nucleus (SCN) of the hypothalamus serves as the regulatory hub, autonomously generating rhythms (Hastings et al., 2019) and perceiving external light signals via intrinsically photosensitive retinal ganglion cells (ipRGCs) (Zhang et al., 2021). This synchronizes rhythmic activities across peripheral tissues—including the liver, kidneys, and hippocampus—forming an integrated central–peripheral regulatory network that maintains homeostasis via hormonal (Blancas-Velazquez et al., 2023; Ramirez-Plascencia et al., 2025), metabolic (Reinke and Asher, 2019), and immune pathways (Zeng et al., 2024). However, modern lifestyles characterized by artificial nighttime illumination and shift work have rendered continuous light exposure a widespread disruptor of circadian organization. Satellite data confirm that light pollution is intensifying globally (Kyba et al., 2017). Such exposure impairs SCN function, suppresses oscillations of core clock genes such as Bmal1 and Clock, and can abrogate rhythmicity entirely (Bumgarner and Nelson, 2021). It also disrupts sleep–wake cycles by inhibiting melatonin synthesis (Schwartz et al., 2009). Critically, this circadian disruption may further trigger inflammatory responses within the central nervous system, particularly through the activation of microglia—the brain’s resident immune cells—thereby exacerbating tissue damage and cognitive dysfunction (Meng et al., 2023). These effects are time-dependent: short-term light exposure induces gene phase shifts, whereas long-term exposure causes irreversible dysregulation of circadian networks, contributing to chronic conditions including obesity, type 2 diabetes, and cognitive decline (Naveed et al., 2024; Schrader et al., 2024). The correlation between cognitive decline and hippocampal dysfunction suggests that continuous light may target the hippocampal circadian system to impair memory.

The hippocampus, a brain region central to learning and memory, relies on finely tuned neural circuit function to support processes such as memory encoding. For instance, NMDA receptors in CA1 help maintain firing rate homeostasis (Ruggiero et al., 2025), underscoring the region’s sensitivity to both internal rhythmic and external metabolic influences. Hippocampal synaptic plasticity exhibits clear circadian variation: long-term potentiation (LTP) varies in induction efficiency across the day (Nakatsuka and Natsume, 2014). At the molecular level, phosphorylation of BMAL1 regulates synaptic localization and plasticity (Barone et al., 2023), intact BMAL1 expression is required for memory retrieval (Hasegawa et al., 2019), and Per1 oscillations correlate with daytime memory consolidation (Bellfy et al., 2023)—confirming that circadian disruption directly compromises hippocampal cognition. Metabolically, the hippocampus is highly vulnerable to fluctuations: neurons depend on glucose availability, and loss of GLUT3 or glycolytic enzymes causes memory deficits (Li et al., 2023). Furthermore, circadian control of mitochondrial oxidative phosphorylation matches energy supply to the demands of synaptic remodeling (Xavier et al., 2016), indicating that metabolic homeostasis is essential for hippocampal function.

Although previous work has linked hippocampal metabolic changes to cognitive impairment—for example, altered cholesterol sulfate levels in rats with radiation-induced memory deficits (Guan et al., 2024)—most studies have relied on static, single-timepoint metabolite measurements, failing to capture dynamic rhythmic profiles. Thus, the role of metabolic rhythm disruption in cognitive decline remains unclear. Time-series metabolomics addresses this gap by sampling across multiple circadian time points (e.g., ZT0, ZT6, ZT12, ZT18) to quantify oscillatory parameters such as phase and amplitude, thereby identifying specific patterns of rhythm disruption. A recent study that profiled eight mouse tissues over 24 h successfully mapped tissue-specific metabolic rhythms and revealed intertissue coordination (Dyar et al., 2018), offering a validated framework for investigating hippocampal metabolic rhythms under continuous light. Therefore, the present study uses a mouse model of short-term continuous light exposure to examine changes in hippocampal clock gene expression, circadian metabolite dynamics, and cognitive behavior. We aimed to systematically characterize the multi-level disruptions induced by continuous light exposure by integrating analyses of hippocampal clock gene expression, circadian metabolite dynamics via time-series metabolomics, and cognitive behavior. Our specific objectives were to identify the core metabolic oscillations disrupted by LL, pinpoint key metabolites and pathways whose rhythmicity is most vulnerable, and explore how these circadian-metabolic disturbances correlate with neuroinflammation and cognitive dysfunction. This integrated approach provides insights into the underlying metabolic mechanisms of light-induced cognitive dysfunction.

Light pollution has emerged as an increasingly prevalent environmental stressor in modern society, and its disruption of the circadian system is linked to an increased risk of various neurological disorders (Voigt et al., 2024). Previous studies monitoring overall activity/rest rhythms in animals have demonstrated significant impacts of light disturbance on behavioral circadian patterns (Walker et al., 2020). In this context, the present study delves further into specific tissues, aiming to systematically investigate how oscillations of the internal “molecular clock” and rhythms of the “metabolic clock” in the hippocampus—a region closely linked to cognition—respond to early-life light exposure. To address this question, we first focused on establishing a short-term exposure model capable of capturing initial pathogenic effects. By systematically comparing continuous light exposure for 2 days (acute), 7 days (short-term), and 35 days (long-term), we found that 7-day (short-term) exposure induced the most pronounced and widespread disruption in the oscillation of core clock genes in the hippocampus, while avoiding complex adaptive changes that might arise from longer-term exposure. Therefore, this study defined “7 days” as the “short-term” exposure window and integrated behavioral analysis, multi-timepoint metabolomics, and molecular biology assays to systematically characterize multi-level disturbances in the hippocampus under short-term continuous light exposure.

Based on the aforementioned model, we found that short-term continuous light exposure first significantly disrupted oscillation patterns of core clock genes in the hippocampus, inducing notable phase shifts and amplitude alterations (Figure 1). Stable rhythmic expression of hippocampal core clock genes serves as a crucial molecular foundation for maintaining synaptic plasticity and memory function (Hasegawa et al., 2019; Barone et al., 2023; Bellfy et al., 2023), which relies on coordinated interactions between the Clock/Bmal1 positive feedback loop and the Per/Cry and Rev-erb α/β negative feedback loops (Bugge et al., 2012). This study discovered that even short-term continuous light exposure significantly disturbed rhythmic expression of hippocampal clock genes, leading to memory impairment in mice. This observation aligns with findings from other circadian disruption models, such as sleep deprivation (Jiao et al., 2022), which also demonstrated cognitive deficits. The functional specificity of the hippocampus as a cognition-related brain region may stem from its dual regulatory requirement—maintaining intrinsic rhythms while responding to central zeitgeber signals (Liu et al., 2024)—potentially rendering it more vulnerable to external light disturbances. Importantly, the vulnerability of hippocampal rhythmic function to short-term continuous light exposure resonates with the finding that “circadian disruption is preferentially associated with reduced hippocampal volume and mediates the risk of cognitive decline” (Luo et al., 2025), further suggesting the hippocampus is a potential early target through which rhythm disruption triggers cognitive impairment.

To investigate the biochemical basis of early functional impairment, we further employed time-series metabolomic analysis. Disruption of core clock genes has been shown to directly regulate circadian oscillations of metabolic genes, thereby driving rhythmic expression of metabolites (Schrader et al., 2024). Our time-series metabolomic analysis confirmed that this top-down regulatory mechanism was systematically disrupted under light interference (Figure 4). PCA revealed clear separation between control and light-exposed groups, indicating an imbalance in overall metabolic homeostasis. Subsequent screening identified 39 differential metabolites, primarily involving amino acids and their derivatives, neuron membrane-related lipids, and neurotransmitter-related metabolites. This suggests selective disturbance of specific metabolic pathways rather than a generalized effect. Among these, elevated levels of metabolites related to neuronal membrane structure hinted at disrupted membrane homeostasis (Yue et al., 2020), while a significant decrease in suberylcarnitine—a key intermediate in mitochondrial β-oxidation—reflected impaired energy metabolism (Guerreiro et al., 2018). Continuous light exposure reduced rhythmic metabolites by 56.9%, and the proportion of metabolites peaking at night dropped from 56.9% to 16%. Given that these metabolites are primarily involved in memory-related processes such as cellular repair and energy storage (Hepler et al., 2022), their sharp decline may directly undermine the metabolic foundation required for cognitive function. Our analysis further pinpointed a highly specific axis of perturbation. From the multitude of rhythmically abnormal metabolites, we identified eight core metabolites with disrupted rhythms. Notably, the rhythmic disturbance of GABA—the major inhibitory neurotransmitter in the hippocampus—was particularly critical (Figure 6C). This is not only because GABA is a core neurotransmitter for maintaining hippocampal excitation/inhibition balance and memory consolidation (Gao et al., 2014; Sohal and Rubenstein, 2019; Wu et al., 2024), but also because its endogenous peak in the control group was precisely timed to the critical period for memory consolidation (ZT12-18) (Maddaloni et al., 2024). In contrast, light treatment caused a significant phase delay (approximately 6.4 h) and a reduction in amplitude (21%). This dysregulation of a core neurotransmitter rhythm within a critical time window likely directly disrupts the precise neurochemical timing and circuit stability necessary for memory consolidation. It is important to emphasize that the current measurements, based on bulk hippocampal tissue, cannot distinguish whether the observed alterations in GABA rhythm stem from an imbalance between synthesis and degradation, specific contributions of neurons versus glial cells, or heterogeneous distribution across hippocampal subregions. Precise characterization of these aspects awaits future investigations utilizing high-resolution techniques such as single-cell sequencing, spatial metabolomics, or in vivo microdialysis.

Importantly, this study identified vitamin B6 metabolism as a significantly disrupted pathway, suggesting it may serve as an upstream regulatory node for GABA phase abnormality. This provides a crucial clue for interpreting how continuous light exposure (LL) disrupts the rhythmicity of hippocampal γ-aminobutyric acid (GABA)ergic transmission. This potential regulatory pathway originates from the global control of metabolic processes by the circadian clock: existing research confirms that core clock proteins (e.g., CLOCK/BMAL1) can rhythmically bind to promoter regions of a vast number of target genes across the genome, with genes related to metabolic pathways being among their core regulatory targets (Koike et al., 2012). This implies that LL-induced circadian disruption in the hippocampus may broadly impair the rhythmic expression of downstream metabolic enzymes, thereby affecting the metabolic homeostasis of vitamin B6. The active form of vitamin B6—pyridoxal-5′-phosphate (PLP)—is an essential cofactor for glutamate decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis; classic biochemical studies clearly establish that the covalent binding of PLP to the active site of GAD is a prerequisite for initiating the enzymatic reaction of GABA synthesis (Ueland et al., 2015). Although the circadian rhythm of GAD activity in the hippocampus has not been directly demonstrated, in the central circadian clock (the suprachiasmatic nucleus, SCN), the synthesis, release, and receptor function of GABA signals are tightly and dynamically coupled with the core clock, providing a key theoretical paradigm for understanding the clock-regulated mode of neurotransmitter synthesis in other brain regions (Albers et al., 2017). Therefore, the LL-induced disruption of vitamin B6 metabolism likely interferes with the rhythmic foundation of GABA synthesis by disturbing the homeostatic supply of PLP. This “metabolism-neurotransmitter” coupling perspective not only precisely links the external stimulus of environmental light with neurochemical changes within the hippocampus but also provides critical molecular evidence for deciphering how light exposure influences neurotransmitter rhythms through time-specific metabolic regulation. It should be emphasized that the proposed regulatory model remains a testable hypothesis, grounded in our correlative findings and established biochemical principles; definitive establishment of causality will require future validation through targeted interventional studies.

The disruption in the vitamin B6 pathway and GABA rhythmicity provides a critical entry point for understanding the complex pathological phenotypes observed in the hippocampus under light exposure. First, at the behavioral level, the phase delay in GABA rhythm is congruent with the observed memory impairment, offering a direct and plausible neurophysiological basis for explaining the impaired recent memory exhibited by mice under continuous light exposure in the Novel Object Recognition and Passive Avoidance tests. We selected these two tests precisely because they effectively capture hippocampal-dependent memory abnormalities sensitive to temporal cues: performance in the Novel Object Recognition test highly depends on the time delay between learning and testing (Clark et al., 2000), while the Passive Avoidance test can accurately reflect memory consolidation within a specific time window (Taubenfeld et al., 2001). Temporal analysis further revealed circadian phase-specific metabolic disturbances: the most significant differences occurred at ZT0—a key time node in entrainment mechanisms (Van Reeth and Turek, 1992)—and at ZT12—a critical window for memory consolidation (Maddaloni et al., 2024). These disturbances correspond to disruptions in time-keeping mechanisms and abnormal energy supply, respectively, thereby explaining from a metabolic perspective “why light exposure preferentially impairs specific cognitive functions.” Second, at the cellular microenvironment level, this concurrent disruption of rhythms and metabolism is closely associated with an imbalance in neuroimmune homeostasis. One important function of the circadian system is to suppress unnecessary inflammatory responses during the rest phase by regulating pathways such as NF-κB (Spengler et al., 2012). Consistent with this theoretical expectation, we synchronously observed transformation of microglia into an activated state and a significant increase in levels of pro-inflammatory cytokines IL-1β and TNF-α in hippocampal tissues with disrupted circadian rhythms—further supporting the notion that circadian disruption may lead to breakdown of neuroimmune homeostasis and uncontrolled inflammatory responses. Notably, microglia, as the brain’s intrinsic immune sentinels, have their physiological functions and immune responses precisely regulated by the endogenous circadian clock (Guzmán-Ruiz et al., 2023). This makes them particularly sensitive to rhythm disturbances caused by light exposure, positioning them as key cells mediating early neuroinflammation. Furthermore, key products of microglial activation—IL-1β and TNF-α—have been confirmed to directly impair hippocampal synaptic plasticity and function (Pribiag and Stellwagen, 2013; Hoshino et al., 2017). This suggests that the inflammatory state we observed likely directly contributes to functional impairment of hippocampal circuits through these mechanisms. Subsequently, at the tissue structure level, the aforementioned functional disturbances are directly reflected in abnormal neuronal morphology. In the light-exposed group, neuronal shrinkage in the hippocampal CA1 region and reduced Nissl bodies in the CA3 region align with pathological changes induced by inflammatory stress. This process may be related to the “phagoptosis” mechanism mediated by activated microglia (Brown and Neher, 2014), wherein microglia can directly phagocytose viable neurons under inflammatory stress, leading to structural damage.

Our findings are suggestive of the possibility that continuous light exposure may trigger a hippocampal damage cascade: circadian rhythm disruption of clock genes may induce systemic metabolic rhythm alterations; abnormal GABA rhythms and vitamin B6 metabolic disturbances may represent key intermediate links, potentially affecting neurotransmitter homeostasis; and these changes may be associated with hippocampal structural abnormalities and cognitive decline. Additional metabolite alterations provide further clues: disrupted fructose-6-phosphate rhythms and enriched starch-sucrose metabolism pathways during ZT12 suggest potential temporal disruption of energy supply during memory consolidation windows (Shao et al., 2018; Landry et al., 2025); abnormal betaine rhythms may influence epigenetic modification processes (Li et al., 2015); and TMAO dysrhythmia coupled with altered butyrate metabolism suggests possible gut–brain axis involvement in neuroinflammation regulation (Matenchuk et al., 2020; Hoyles et al., 2021; Connell et al., 2022). Collectively, these findings support the concept that light exposure affects the hippocampus through multi-pathway, multi-level systemic perturbations, where temporally and functionally interconnected abnormalities form a potential network of action.

Furthermore, this study has several limitations. First and foremost, our findings establish correlative links between circadian-metabolic disruption and functional impairment; however, causal relationships remain to be definitively established. Future interventional studies (e.g., modulating vitamin B6 metabolism or GABAergic signaling) are needed to test the proposed pathways. Second, behavioral testing was conducted at a fixed circadian phase (ZT2–ZT6) to control for time-of-day variation. While this design robustly confirms a basal memory decline, it cannot reveal whether the endogenous circadian rhythm of memory performance itself is altered. Future studies employing multi-timepoint testing across the 24-h cycle are needed to address this question. Third, although we employed a short-term exposure model to capture initial disruptions, the observed effects could, in part, involve non-specific stress responses. Future work incorporating direct stress markers (e.g., corticosterone) would help distinguish circadian-specific effects from general stress. Finally, this study was conducted in male mice. Generalizability to females, whose circadian and metabolic systems may respond differently, requires explicit investigation in future studies.

In summary, this study demonstrates that short-term continuous light exposure is associated with multi-level disruptions in the mouse hippocampus. These disruptions include desynchronization of core circadian clock genes, systemic loss and phase alteration of metabolic rhythms, onset of neuroinflammation, neuronal structural abnormalities, and impairments in spatial and aversive memory. Notably, among the observed metabolic disturbances, the rhythm of the neurotransmitter GABA was significantly phase-delayed and dampened, and the vitamin B6 metabolism pathway was prominently dysregulated. Concurrent alterations were also noted in metabolites related to energy metabolism, methylation, and the gut-brain axis. Collectively, these results delineate a correlative framework in which environmental circadian disruption, metabolic rhythm disturbances, neuroinflammation, and cognitive decline co-occur in the hippocampus following short-term continuous light exposure. Future studies employing interventional designs are needed to establish causal relationships among these pathways. This work provides novel metabolomic clues and a descriptive foundation for further investigating the health risks associated with light pollution.

The author(s) declared that financial support was not received for this work and/or its publication.

The original contributions presented in the study are included in the article/, further inquiries can be directed to the corresponding authors. Supplementary Material

The animal study was approved by the Committee on the Ethics of Animal Experiments of the Academy of Military Medical Science. Affiliation: Academy of Military Medical Science. The study was conducted in accordance with the local legislation and institutional requirements.

XW: Software, Writing – original draft, Formal Analysis, Conceptualization. GZ: Methodology, Validation, Conceptualization, Writing – original draft. SW: Writing – review and editing, Methodology, Investigation, Software, Visualization. YH: Writing – review and editing, Conceptualization, Investigation, Validation. YZ: Conceptualization, Writing – review and editing, Investigation. LZ: Investigation, Writing – review and editing, Conceptualization. XL: Writing – review and editing, Supervision. JL: Writing – review and editing, Supervision. DY: Supervision, Writing – review and editing.

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that generative AI was not used in the creation of this manuscript.

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