The circadian regulation of LH by the hypothalamic-pituitary-gonadal (HPG) axis serves as a key signal for corpus luteum formation and ovulation, as shown in Figure 2, which is essential for the proper progression and coordination of reproductive processes. In the hypothalamus, kisspeptin neurons stimulate Gonadotropin-releasing hormone (GnRH) neurons, which release GnRH onto gonadotropes in the anterior pituitary. In response to GnRH, gonadotropes release LH and FSH into the circulation, allowing these hormones to act on the gonads to stimulate gametogenesis and sex steroid production. When the follicles are mature, the level of estrogen released reaches a threshold level, which then becomes an activator to kisspeptin neurons in the anteroventral periventricular nucleus (AVPV). The increased activity and release of kisspeptin by AVPV kisspeptin neurons onto GnRH neurons results in a surge of GnRH, which prompts a surge of LH, and then ovulation (Wang and Moenter, 2020). The VIP acceptor two is expressed on GnRH neurons and VIP neurons located in the SCN possess the capacity to project directly onto GnRH neurons (Van der Beek et al., 1994; An et al., 2011). The absence of VIP leads to a reduction and delay in the LH surge, consequently resulting in impaired ovulation and reduced fertility in mice (Harney et al., 1996; Loh et al., 2014; Hoffmann et al., 2021). AVP neurons in the SCN shell project to AVPV kisspeptin neurons in rodents by vasopressin receptor 1a (V1a), and AVP robustly stimulates kisspeptin neuron firing (Williams et al., 2011; Piet et al., 2015). In SCN-lesioned animals, intracranial injection of AVP in the late afternoon rescues the LH surge through V1a (Palm et al., 1999; Miller et al., 2006). Neuromedin U receptor type 2 (NMU2R), the receptor for NMS, is widely expressed in the hypothalamus and anterior pituitary, particularly in melanocyte-stimulating hormone (MSH) neurons (Crown et al., 2007; Yang et al., 2010). Evidence suggests that NMS regulates luteinizing hormone (LH) secretion by acting on MSH neurons in pigs. Additionally, the administration of exogenous NMS increases serum LH levels in female rats, further supporting the regulatory role of NMS in LH secretion (Vigo et al., 2007; Yang et al., 2010).

E₂ is the main hormone secreted by ovarian granulosa cells and plays an important role in embryo implantation. The steroidogenic acute regulatory protein (STAR) promotes the transport of cholesterol from the outside to the inside of the mitochondrial membrane. Under the catalysis of cytochrome P450 family 11 subfamily a member 1 (CYP11A1), cholesterol undergoes a side-chain cleavage reaction to generate pregnenolone. Pregnenolone is then converted into dehydroepiandrosterone (DHEA) under the action of 17-hydroxylase (CYP17A1). DHEA is catalyzed by 3β-hydroxysteroid dehydrogenase (3β-HSD) to produce androstenedione. Androstenedione is further converted into estrone under the action of aromatase cytochrome P450 family 19 subfamily a member 1 (CYP19A1). Estrone can be further transformed into estradiol with stronger activity under the action of 17β-hydroxysteroid dehydrogenase (17β-HSD) (Wallach et al., 1996). Accumulating evidence indicates that the circadian clock exerts regulatory control over E₂ signaling. The knockdown of Clock genes Bmal1 or Clock via small interfering RNA led to a reduction in the expression of StAR, Cyp11a1, and Cyp19a1, accompanied by a decrease in E₂ content within granulosa cells. Conversely, the knockdown of Per2 enhanced StAR expression and augmented E₂ production. This may be the reason why PER2 is a BMAL1: CLOCK repressor (Shimizu et al., 2011; Wang et al., 2017). REV-ERBα further diminished estrogen secretion in ovarian granulosa cells through a direct interaction with the RORE region of the Cyp19a1 promoter, which in turn suppressed Cyp19a1 expression. Simultaneously, REV-ERBα could also act on the RORE region of the Bmal1 promoter to curtail its expression and undermine Bmal1 function, consequently leading to a reduction in E₂ secretion (Cho et al., 2012; Wang et al., 2022). E₂ is primarily secreted by ovarian granulosa cells (GCs), and the proliferation, apoptosis and autophagic processes of GCs can influence E₂ production. A study utilizing RNA-seq analysis on GCs with CLOCK overexpression successfully identified Ankyrin repeat and suppressor of cytokine signaling box-containing 9 (ASB9) as a differentially expressed gene, which is involved in cellular growth and differentiation processes (Benoit et al., 2019; Huang et al., 2023). Experimental findings demonstrated that ASB9 is a direct target gene of CLOCK, through which CLOCK increases the population of cells in the G1 phase, reduces the number of cells in the G2 phase, and suppresses the viability of GCs (Huang et al., 2023). Circadian rhythms are not only involved in the proliferation of GCs but also play a role in GCs apoptosis. In an experiment involving the knockdown of Bmal1 in porcine GCs, the phosphoinositide 3-kinase (PI3K)/protein kinase b (Akt)/mechanistic target of rapamycin (mTOR) signaling pathway was inactivated, indicating the onset of apoptosis in GCs. This finding was further corroborated by flow cytometry analysis (Wang et al., 2017). Autophagy, a tightly regulated lysosomal degradation pathway, is essential for clearing long-lived proteins and damaged organelles. Dysregulation of this process can have severe cellular consequences (Klionsky and Emr, 2000; Kroemer et al., 2010). Autophagy-related 5 (Atg5) is a CCG regulated by Rev-erbα, which negatively modulates Atg5 expression, leading to autophagy dysregulation in mice GCs (Zhang et al., 2022). Nuclear receptor coactivator 4 (NCOA4) is a cargo receptor responsible for autophagy-dependent ferritin degradation (Mancias et al., 2014). NCOA4-mediated ferritinophagy maintains intracellular iron homeostasis by facilitating ferritin iron storage or release according to demand. NCOA4 deletion inhibits ferroptosis by blocking ferritinophagy and ferritin degradation (Liu et al., 2020). In human ovarian GCs, Cry1 modulates NCOA4-mediated ferritinophagy by regulating NCOA4 ubiquitination and subsequent degradation. Furthermore, treatment with KL201, a Cry1 stabilizer, effectively suppresses ferritinophagy (Ma et al., 2024). In summary, as synthesized in Table 1, the circadian system plays a fundamental role in regulating both the initiation and termination of ovarian GCs functions, critically influencing their capacity to secrete E₂.

P₄ is the main hormone secreted by the corpus luteum (CL). The source of cholesterol for steroidogenesis in ovarian luteal cells depends on circulating plasma lipoproteins, de novo synthesis, and utilization of intracellular cholesterol ester stores. STAR facilitates the transport of cholesterol from the outer to the inner mitochondrial membrane, serving as the rate-limiting step in progesterone synthesis. CYP11A1 catalyzes the conversion of cholesterol to pregnenolone, which passes into the smooth endoplasmic reticulum where it is converted to progesterone by 3β-HSD. P₄ then diffuses out of the luteal cell to be transported to the target tissues (Yakin et al., 2023). Accumulating evidence suggests that the circadian clock exerts significant regulatory control over P₄ signaling pathways. In an experiment involving mice subjected to constant light, the researchers discovered that mice with circadian rhythm disruptions induced by continuous light exposure exhibited lower StAR and serum P₄ levels (Li et al., 2023). Continuous light exposure and a 6-h phase shift every 3 days of light exposure led to a reduction in serum P₄ levels in ruminants (Gao et al., 2016; Suarez-Trujillo et al., 2022). Specifically, it is the peripheral clock proteins, such as BMAL1, that are operative. BMAL1 global knockout female mice were found to be infertile (Ratajczak et al., 2009; Boden et al., 2010). However, when additional P₄ was continuously administered from day 3.5 to day 6.3 post-fertilization, embryo implantation and pregnancy establishment occurred. During pregnancy, at day 3.5, BMAL1 global knockout mice exhibited lower levels of StAR and serum P₄ (Ratajczak et al., 2009). To validate the role of BMAL1 in the modulation of progesterone secretion within the ovary, Liu has ascertained this role through the specific knockout of BMAL1 in steroidogenic cells and subsequent ovarian transplantation (Liu et al., 2014).

In addition, as delineated in Figure 2, circadian rhythms also regulate luteinization and luteolysis. Luteinization is the basis for the secretion of progesterone by the corpus luteum. Ovulation leads to the establishment of a local hypoxic microenvironment. This hypoxic condition triggers a significant upsurge in hypoxia-inducible factor 1α (HIF-1α) (Zhang et al., 2011). Subsequently, HIF-1α associates with HIF-1β to form a heterodimer (HIF-1). The formed dimer then binds to the cis-hypoxia response element (HRE) located within the VEGF promoter region, thereby facilitating and enhancing VEGF mRNA expression (Kazi et al., 2005). The circadian-expressed CLOCK and PER2 functions as an effector molecule, which is involved in promoting the recruitment of HIF-1 to the HRE region of the VEGF promoter (Tang et al., 2015; Kobayashi et al., 2017). However, in zebrafish, PER2 was shown to inhibit VEGF (Jensen et al., 2012). In nucleus pulposus cells, the suppression of BMAL1 and RORα leads to the decrease of the expression of HIF-1 and VEGF (Suyama et al., 2016). BMAL1 even more has been demonstrated to act as a transcription factor in facilitating VEGF expression (Jensen et al., 2012; Guo et al., 2021; Zhang et al., 2023). The inhibitory proteins PER and CRY heterodimerize into the nucleus and directly interact with BMAL: CLOCK to inhibit its transcriptional function (Michael et al., 2017; Rosensweig et al., 2018). The functions of BMAL1 and PER2 are thus in opposition, and we are confident that PER2 regulates the role of HIF-1, but the role of PER2 in the regulation of VEGF is debatable. It is routinely assumed that PER2 regulates VEGF expression through HIF-1, but due to the inhibitory effect of PER2 on BMAL1, the pro-VEGF expression of BMAL1 is weakened, and the end result is attenuated VEGF (Koyanagi et al., 2003; Su et al., 2017). This indicates that Vegf is a CCG, and several studies have also corroborated this notion (Frigato et al., 2009; Wharfe et al., 2011; Yang et al., 2015). Although research on the regulation of VEGF by circadian genes during ovarian luteinization is currently lacking, the significance of circadian regulation of VEGF has been established by numerous investigations. Additionally, there is evidence indicating that BMAL1 coincides with the HIF-1 peak during luteinization (Kobayashi et al., 2018). This evidence suggests that circadian genes play a crucial role in modulating angiogenesis during luteinization. Luteal regression is essential in triggering the development of a new follicle and restarting the estrous cycle. During the stage of diestrus, the CL regresses, losing its capacity to produce P₄ and under goes structural involution. Proapoptotic and antiapoptotic factors have been implicated in structural luteal regression. The pro-apoptotic factors, namely, Fas, FasL, and Bax, as well as the anti-apoptotic factor Bcl-2, are rhythmically expressed within the ovary. Additionally, the promoter regions of these factors all possess BMAL1-binding E-box sequences. During luteal phase apoptosis, the peak expression of these factors followed the peak in BMAL1 expression (De La Vega et al., 2018).

E₂ and P₄ regulated by the HPG axis, play crucial roles in endometrial receptivity and decidualization during embryo implantation. As shown in Table 1, these hormones are essential for the implantation process. Furthermore, they are themselves regulated by circadian rhythms, thereby mediating the circadian coordination of embryo implantation through this bidirectional regulatory mechanism.

Bone morphogenetic proteins (BMPs), which belong to the transforming growth factor-β (TGF-β) superfamily, are implicated in a diverse range of cellular functions, such as proliferation, differentiation, and remodeling (Shimasaki et al., 2004). The BMP family, comprising BMP2, BMP4, BMP6, and BMP7, exhibits spatiotemporal expression in the mouse uterus during the successive phases of implantation. BMP2 is abundantly expressed within the decidual area encircling the site of blastocyst attachment and assumes a crucial function in decidualization (Ying and Zhao, 2000; Li et al., 2007). Emerging evidence indicates that the circadian clock exerts stable and significant regulatory effects on BMPs. In both humans and rodents, UESCs undergo proliferation and differentiation into decidual cells. In vitro decidualization was induced by medroxyprogesterone acetate and 2-O-dibutyryl cAMP (He et al., 2007). It was observed that the knockdown of Bmal1 led to a downregulation of Rev-erbα expression and an upregulation of Bmp2/4/6 expression. Subsequent to the application of a REV-ERBα antagonist, the expression of Bmp1/2/4/6/7/8a was enhanced. These findings imply that Rev-erbα is a significant circadian clock gene that governs the BMP family and, as shown in Figure 4, functions as a transcription factor by binding to the RORE regions of the Bmp2 and Bmp4 promoters to modulate their expression (Tasaki et al., 2015).

Growth/differentiation factors (GDFs) are members of the TGF-β superfamily, and they are involved in a variety of cellular functions and biological processes such as cell proliferation, differentiation, and remodeling (Whitman, 1998). Gdf10 and Gdf15 are ubiquitously expressed throughout the uterus, especially during the crucial period of embryo implantation. This widespread expression pattern strongly suggests their significant and active roles in the implantation process (Fairlie et al., 1999; Zhao et al., 1999). Notably, both Gdf10 and Gdf15 exhibit a remarkable and significant increase in expression levels during the decidualization of UESCs, thereby further implying their essential contributions to the decidualization process. The circadian clock exhibits robust regulatory control over GDFs. When decidualized UESCs are treated with a REV-ERBα inhibitor, it leads to an upregulation in the expression of both Gdf10 and Gdf15. Moreover, through further chromatin immunoprecipitation analysis, it has been revealed that REV-ERBα, as demonstrated in Figure 4, exerts its inhibitory effect on Gdf10 and Gdf15 by directly binding to their respective promoters (Zhao et al., 2016).

Prostaglandins (PGs) are produced through the hydrolysis of membrane phospholipids by cytoplasmic phospholipase A2 to release arachidonic acid, which is converted to PGs by Cyclooxygenase-2 (COX2) and PG endoperoxide H synthase. PGs intermediate the functions of the corpus luteum, participate in maternal–fetal interface immunomodulation and pregnancy identification, and stimulate angiogenesis during early pregnancy (Ye et al., 2021). PGs can also regulate myometrium relaxation and contraction via PG transporters and receptors, thus affecting blastocyst transportation and adhesion reactions of the endometrium–trophoblast, ultimately regulating the distribution of the implanted embryos in the uterus (Blitek and Szymanska, 2020). COX serves as the rate-limiting enzyme in the synthesis of PGs. Mice lacking COX-2 display unsuccessful embryo implantation and defective uterine decidualization (Lim et al., 1997). COX-2 is also a time-regulated gene, exhibiting robust rhythmicity at rat D3.5-4.5 of pregnancy. The suppression of Bmal1 expression in rat UESC led to a reduction in the expression levels of Cox-2 and PGE2. It was highly expected that (Figure 3) the inhibition of REV-ERBα, which is a repressor of Bmal1, would enhance the expression of Cox-2 (Chen et al., 2013; Isayama et al., 2014; 2015; Zhao et al., 2021). This effectively illustrates the precise and effective regulation of the prostaglandin synthesis pathway by the circadian clock loop.

It is widely acknowledged that the interleukin (IL)-6 family, a group of cytokines, holds significant importance during embryonic implantation (Dimitriadis et al., 2005). The IL-6 family encompasses several cytokines, such as leukemia inhibitory factor (LIF), IL-6, IL-11, and neurotrophic factor. Among the cytokines that have been investigated, LIF is most relevant to implantation (Kimber, 2005; White et al., 2007). The expression of LIF exhibits a biphasic pattern on day 4, initially appearing in the uterine glands and subsequently in the stromal cells surrounding the blastocyst during the attachment reaction (Song et al., 2000; Ni et al., 2002). This expression profile implies that LIF has dual functions, being involved in uterine preparation initially and then in the attachment reaction (Stewart et al., 1992; Song et al., 2000). Female mice with a deficiency in LIF experience implantation failure, and this defect can be rescued by supplementation with LIF (Stewart et al., 1992). In addition to LIF, IL-6 is another crucial cytokine for successful pregnancy. In mice, it is secreted by the epithelial and stromal cells in the uterus and is regulated by ovarian steroid hormones (Prins et al., 2012). Mice lacking IL-6 display impaired implantation and a delayed onset of labor, leading to adverse pregnancy outcomes (Robertson et al., 2010). In humans, IL-6 is mainly produced by endometrial epithelium and stromal cells in a cyclic manner. The levels of IL-6 are relatively low during the proliferative phase and increase steadily during the secretory phase, suggesting its important role during implantation (Jasper et al., 2007; Champion et al., 2012). Melatonin (MT) represents a clock control hormone that is secreted by the pineal gland, ovary, and placenta and plays a crucial role in modulating endometrial receptivity and immunity (Cajochen et al., 2003; Wu et al., 2017). MT exerts its effects via two receptors, MT1/2, which are expressed in a circadian manner (Wu et al., 2017). In a research study on human endometrial receptivity, it was proposed that MT could enhance endometrial receptivity through the nuclear factor kappa B (NF-κB) and apoptotic pathways. Concurrently, MT also stimulated the expression of LIF and IL-6 in the endometrium (Guan et al., 2022; Zheng, 2022).

Studies have revealed that the internal time-keeping system circadian clock genes are responsible for driving the circadian rhythms evident in the immune system. For instance, the recruitment of immune cells (such as monocytes, neutrophils, and lymphocytes), antigen presentation, lymphocyte proliferation, and cytokine gene expressions occur in accordance with a 24-h daily rhythm, thereby initiating an acute response to infection (Nakao, 2014). In mouse aortic endothelial cells, the knockdown of the Clock gene led to a significant downregulation of LIF expression (Jiang et al., 2018). In mice with specific deletion of BMAL1 in myeloid cells, the temporal variations in serum IL-6 following lipopolysaccharide (LPS) challenge were not observed. BMAL1 exerts a downstream effect by activating the transcription of the nuclear receptor Rev-erbα, and REV-ERBα, in turn, inhibits BMAL1. Consequently, in Rev-erbα-deficient mice, these rhythmic immune responses to LPS were abolished. This finding implies that there is a connection among BMAL1, REV-ERBα, and the production of IL-6 in macrophages upon LPS challenge (Gibbs et al., 2012). This finding (Figure 3) is buttressed by the observed inhibition of IL-6 expression by REV-ERBα in bovine endometrial epithelial cells (Yang W. et al., 2024). Finally, Cry is also involved in uterine receptive immunity. Macrophages derived from Cry1/2 knockout mice exhibited an enhanced secretion of IL-6 (Narasimamurthy et al., 2012).

The glucocorticoid hormone cortisol is a primary product of the hypothalamic-pituitary-adrenal (HPA) axis, a key biological stress response system. The effects of glucocorticoids are mediated by the glucocorticoid receptor (GR), which translocates to the nucleus in a ligand-dependent manner and acts as a transcription factor to regulate gene expression (Kumar and Thompson, 2005). Alterations in cortisol levels have been associated with impaired trophoblast implantation and dysfunctional activity. In first-trimester trophoblast cell line, Sw.71, the addition of cortisol was shown to inhibit trophoblast cell invasion, thereby suppressing the implantation process (Smith et al., 2017; Kisanga et al., 2018). Cortisol exhibits a distinct endogenous circadian rhythm, modulated by sleep/wake cycles, dietary intake, and physical activity. Shift workers and night-shift nurses demonstrate significantly lower cortisol levels and display abnormal circadian rhythmicity due to circadian misalignment (Harris et al., 2010; Yang Z. et al., 2024). Cortisol demonstrates a strong correlation with peripheral circadian clock genes. Bmal1-knockout macaque monkeys exhibited significantly elevated cortisol concentrations accompanied by diminished oscillation amplitude, indicating disrupted glucocorticoid circadian regulation (Qiu et al., 2019). Exogenous cortisol administration significantly upregulated Per1 expression in human peripheral blood mononuclear cells (PBMCs), while concurrently inducing phase shifts in the circadian oscillations of Per2, Per3, and Bmal1 transcriptional patterns (Cuesta et al., 2015). Collectively, these findings suggest that cortisol is under circadian regulation and plays a regulatory role in embryonic implantation. Furthermore, the investigation of the circadian rhythm-cortisol-embryonic implantation axis and cortisol-mediated regulation of clock genes during implantation warrants further investigation to establish more direct mechanistic evidence.

Ghrelin, an appetite-stimulating hormone produced by gastric P/D1 cells, and leptin, an appetite-suppressing hormone secreted by white adipocytes, have been demonstrated to participate in human endometrial decidualization processes (Cervero et al., 2004; Tawadros et al., 2007). Ghrelin demonstrates substantial upregulation during decidualization and facilitates human UESCs decidualization through activation of the growth hormone secretagogue receptor signaling pathway (Tanaka et al., 2003; Tawadros et al., 2007). Diet-induced obese murine models exhibit delayed decidualization and disrupted leptin signaling (Walewska et al., 2024). Exogenous leptin supplementation significantly enhanced embryo implantation efficiency in both in vivo and in vitro experimental systems through leptin receptor-mediated janus kinase (JAK)/signal transducer and activator of transcription (STAT) activation pathways (Yang et al., 2006; Barnes et al., 2020). In healthy adults under energy-balanced conditions, circulating ghrelin levels exhibit a 24-h oscillatory pattern synchronized with circadian rhythms (Cummings et al., 2002). In vitro experiments have also revealed that Ghrelin promotes decidualization since, the peptide enhances the production of insulin-like growth factor binding protein-1 (IGFBP-1) by human UESCs (Tawadros et al., 2007). Sleep deprivation and circadian misalignment contribute to obesity pathogenesis, primarily through metabolic dysregulation characterized by elevated ghrelin concentrations and suppressed leptin levels, thereby promoting positive energy intake (Wright, 2009; Qian et al., 2019). Genetic knockout models of Bmal1, Clock, Per2, or Rev-erbs exhibited elevated leptin levels (Turek et al., 2005; Yang et al., 2009; Kennaway et al., 2013; Adlanmerini et al., 2021). Leptin administration downregulated Cry1 expression while concurrently upregulating Rev-erbα transcriptional activity (Vieira et al., 2012; Wei et al., 2021). In contrast, Bmal1-knockout mice displayed reduced ghrelin levels (Laermans et al., 2015). Accumulating evidence demonstrates that ghrelin and leptin exhibit robust bidirectional interactions with the circadian regulatory system. These metabolic hormones are rhythmically modulated by circadian oscillators to participate in embryo implantation processes.

Insulin is secreted by pancreatic β-cells in response to fluctuations in blood glucose levels (Tokarz et al., 2018). During embryo implantation, insulin suppresses the production of IGFBP-1 in UESCs, which is recognized as a biochemical marker of decidualization. Consequently, hyperinsulinemic conditions are postulated to disrupt normal metabolic homeostasis in the endometrium, compromising implantation success through dysregulation of decidualization-related molecular pathways (Giudice et al., 1992). Hyperinsulinemia represents a hallmark metabolic aberration in polycystic ovary syndrome (PCOS). Notably, PCOS patients exhibit concomitant insulin resistance that downregulates glucose transporter 4 (GLUT4) expression, subsequently impairing glucose transporter activity. This metabolic dysfunction manifests as reduced cellular glucose uptake capacity and abnormal glucose metabolism homeostasis, ultimately contributing to aberrant endometrial differentiation and compromised embryo implantation competence (Zhai et al., 2012). Insulin signaling exhibits a close interplay with circadian rhythms. Shift workers demonstrate reduced insulin sensitivity. The SCN governs 24-h rhythmicity in blood glucose concentrations, with SCN-lesioned mice displaying abolished circadian glucose regulation (Fleur et al., 1999). Genetic ablation of core clock components, including Bmal1, Clock, or Cry1/2, induces hyperglycemia in murine models (Zhang et al., 2010; Reinke and Asher, 2019). Muscle-specific Bmal1 knockout mice manifest impaired muscle insulin sensitivity (Harfmann et al., 2016). Notably, adenovirus-mediated Cry1 overexpression enhances systemic insulin sensitivity in experimental animals (Zhang et al., 2010). Circadian regulation demonstrates a critical mechanistic connection with insulin homeostasis, with particular pathophysiological implications for insulin resistance. Emerging evidence implicates circadian disruption in compromised embryo implantation processes through its modulatory effects on insulin resistance pathways.

As summarized in Table 2, circadian rhythms regulate multiple implantation-critical factors—including BMP2/4, GDF10/15, COX2, LIF, IL-6, Cortisol, Ghrelin, Leptin and Insulin—through direct modulation of key processes such as embryo adhesion, trophoblast invasion, endometrial receptivity, and decidualization, thereby demonstrating their essential role in orchestrating successful embryo implantation.

It is well-established that sunlight exposure facilitates vitamin D synthesis, wherein ultraviolet B (290–320 nm, UVB) radiation converts cutaneous 7-dehydrocholesterol (7-DHC) into vitamin D3 through photochemical reactions. Vitamin D exerts pleiotropic physiological functions, with critical roles in reproductive physiology particularly in embryo implantation. Clinical evidence indicates vitamin D deficiency directly contributes to implantation failure (Halloran and Deluca, 1980). Notably, vitamin D supplementation in normally cycling mice significantly enhanced embryo implantation rates (Lee et al., 2024). The skin serves not only as the primary site of vitamin D synthesis but also as a key model system for circadian clock regulation, with multiple physiological skin processes exhibiting circadian rhythmicity. UVB irradiation was found to entrain rhythmic expression of Bmal1 and Per2 in human HaCaT keratinocytes (Lamnis et al., 2024). Emerging evidence further links vitamin D status to systemic circadian homeostasis: Vitamin D depletion induces hepatic clock gene dysregulation, characterized by downregulated Bmal1, Clock, Per2, and Cry1/2 mRNA levels at ZT1, contrasted by paradoxical upregulation of these transcripts at ZT13 (Li R. et al., 2024). Conversely, vitamin D supplementation amplified the amplitude of Per1:luc circadian oscillations and enhanced rhythmic precision in human bone marrow stromal cells (BMSCs) (Hassan et al., 2017). While accumulating evidence suggests that vitamin D exhibits significant entrainment relationships with circadian clocks, the regulatory role of circadian rhythms in mediating vitamin D’s effects during embryo implantation warrants further investigation.

There is evidence demonstrating that female Hoxa10 ^−/− mice experience infertility and implantation failure when wild-type embryos are transferred into them. Meanwhile, the expression of HOXA10 is remarkably enhanced during implantation, and specific inhibition of HOXA10 in the endometrium leads to a decrease in the number of implanted embryos. Besides its role in receptivity, HOXA10 also plays a significant part in endometrial decidualization. It is prominently expressed during decidualization. In 40% of the mice with HOXA10 null mutation, although implantation was successful, local hemorrhage at the implantation site, disordered embryos and empty decidua were observed. These results have also been validated in vitro (Benson et al., 1996; Lim et al., 1999; Modi and Godbole, 2009). In a research study carried out on hamsters, it was observed that a daily light exposure regimen with a longer light period (light/dark cycle of 16/8) led to a diminished expression level of HOXA10 and a greater frequency of uterine abnormalities when contrasted with the normal light exposure setting (light/dark cycle of 12/12). The identical outcomes were also manifested in mice that were exposed to continuous light (Das et al., 2022b; 2022a). Currently, there is a lack of reports regarding the specific mechanism by which particular clock genes act on HOXA10. However, based on existing reports and the promoting effect of MT on endometrial HOXA10, HOXA10 is clearly influenced by circadian rhythms (Guan et al., 2022).

Previous studies have clearly manifested that adhesion molecules are of crucial importance in the intimate and essential interaction between the blastocyst and the uterine luminal epithelial cells. Integrins, which are heterodimeric cell surface glycoprotein receptors composed of α and β subunits, serve as mediators for cell adhesion, cell migration, signal transduction, and gene expression (Hynes, 2002; Calderwood, 2004). The apical localization of integrin β3 has been thoroughly documented and recorded throughout the entire course of early pregnancy, accompanied by a significant increase precisely at the time of implantation in the mouse. During the critical moment of implantation, integrin β3 was observed to dissociate from focal adhesions. Simultaneously, integrin β3 demonstrated an augmented presence along the apical membrane of uterine luminal epithelial cells. This particular manifestation suggests that integrin β3 potentially has a substantial and influential role in the intricate process of embryo attachment (Kaneko et al., 2011b; 2011a). Integrin β5 demonstrates a circadian rhythm within retinal pigment epithelial cells and is modulated by Bmal1 in human muscle tissue. In the case of mice, the expression of integrin β3 in platelets was diminished subsequent to the knockdown of Rev-erbα (Milićević et al., 2019; Shi et al., 2022). To date, the interaction between the circadian clock and integrin β3 remains unreported in uterus. Nevertheless, during embryo implantation, the pathological decrease in integrin β3 was reversed upon the addition of supplemental melatonin (Guan et al., 2022). Based on these observations, we hypothesized that integrin β3 may possess the potential to exhibit chronobiological effects.

In addition to vitamin D, HOXA10, and integrins, numerous potential factors regulated by circadian rhythms are involved in embryo implantation. Including P-selectin (Burrows et al., 1994; Qin and Deng, 2015), E-cadherins (Riethmacher et al., 1995; Li et al., 2018) and matrix metalloproteinases 9 (Bai et al., 2005; Li D. et al., 2024), While their roles in the embryo implantation process are well-established and circadian regulation has been evidenced, their specific mechanisms of influence during intrauterine implantation require further investigation.