Rheumatoid arthritis (RA) is a chronic, systemic autoimmune inflammatory disease that primarily affects synovial joints and may result in progressive cartilage destruction, bone erosion, functional disability, and impaired quality of life. The disease develops through complex interactions among genetic susceptibility, environmental factors, immune dysregulation, and inflammatory mediators. Persistent activation of innate and adaptive immune responses within the synovium promotes infiltration of inflammatory cells and proliferation of fibroblast-like synoviocytes, leading to formation of an aggressive inflammatory tissue that progressively damages cartilage and subchondral bone [1]. Cytokines have a major role to play in this regard wherein Tumor Necrosis Factor-alpha (TNF-α), Interleukin-1 Beta (IL-1β) and Interleukin-6 (IL-6) have been found to be the major agents in promoting synovial inflammation and joint destruction [2]. Despite the remarkable progress made in the understanding and treatment of RA, its pathophysiology remains wide open. The efficient application of biological DMARDs - disease modifying antirheumatic drugs and targeted synthetic/AIDS therapies have shown the ability to enhance disease control by inhibition of some specific inflammatroy pathways. Due to that there is huge inter patient variability of the response to treatment and persistence of inflammation may continue despite therapy [5]. Again, experience shows that the inflammation might remain persistent despite antigens/practitioners/clinicians do everything to treat the patient. These have, in turn, motivated systematic investigation of additional biological airways involved in immunoregulation, oxidative stress, the endocrine system and circadian clock and control of the inflammatory process [3]. One hormone that is increasingly being examined in the context of inflammation regulation is melatonin. Melatonin [N-acetyl-5-methoxytryptamine] is a small indole molecule primarily synthesized and secreted by the pineal gland and predominantly around the onset of the dark phase of the circadian cycle. This synthesis is driven by the environmental triggers such as changes in the light and the biological clock itself. Melatonin, even though it is mostly known for its role in sleep and circadian rhythms, has been shown to have antioxidant, immunomodulatory, and anti-inflammatory effects as well [4]. Melatonin can affect immune cells and even the genesis of oxidative stress, signal transduction within cells, and the function of cell subsets responding to inflammation. Based on these features, it would seem reasonable to study the prospects of its involvement in autoimmune inflammatory diseases including RA [5]. Of particular interest is the connection between melatonin and RA since the phenomenon of RA presents itself with characteristics, which are most importantly the clinical manifestations and joint changes cycling in a daily fashion. A peak in pain, inflammation and stiffness of the joints is often seen in the early morning. This observation is consistent with increases in anti-inflammatory and proinflammatory cytokines as well as neuroendocrine hormones during the night, and their reduction during the morning hours [6]. For instance, concentrations of IL-6 go up at night and early in the morning while that of endogenous cortisol, which has an extensive anti-inflammatory effect, follows a different curve [7]. Furthermore, concentrations of melatonin too increase during the dark period and attain peak levels at night. It has been noted in studies that have been carried out that patients have rheumatoid arthritis have changed levels of melatonin compared to healthy individuals with raised concentrations and marked nocturnal peaks indicating a probable defect in melatonin rhythms accounting for the circadian incept of the inflamed state characteristics of arthritis [8]. Nevertheless, the roles of melatonin in the different phases of RA are deeply complex and may even be bidirectional. It is reasonable to conclude sore the potential anti-inflammatory properties of melatonin because more ancient studies and some animal research have suggested that melatonin increased immune reactivity in predisposed individuals possibly through augmentation of the activity of inflammation related mediators [8]. On the contrary, more current experimental issuance shows that melatonin suppresses TNF-α and IL-1β expression by PI3K/AKT, ERK, NF-κB-related signaling cascades (in case of human rheumatoid synovial fibroblasts) which may indicate the drug as inhibiting certain inflammatory responses. Similar effects on inflammatory responses were found in mouse experimental arthritis with confirmed reductions in synovial inflammation, cartilage degradation and bone erosion [9]. In this regard, the melatonin can act differently in respect to concentration, time of administration, activation of receptors, the presence of inflammatory processes, and the surroundings of cells. It is also difficult to base on the clinical arguments. One study, published in a randomized controlled design, proved the efficacy of melatonin in reducing several markers of oxidative stress with special emphasis on malondialdehyde and other indicators, however, no significant reduction in the disease activity of RA as per DAS was observed in patients taking melatonin compared to patients taking placebo [10]. Advancing the above argument through the study of melatonin supplementation in rheumatic conditions, interestingly the sources available have pointed out that melatonin does not demonstrate any encouraging evidence of benefit for RA therapeutics. So, the association of melatonin in RA is still unproven and needs further investigations. It may also be important to assess the levels of circulating melatonin and relate it with several circulating inflammatory molecules and the activity of the disease conditions in order to understand the contribution of the neuroendocrine mechanisms involved in RA, whether it is a biomarker, a contributor, an intermediary effect or even a possible adjunct of therapy in this autoimmune disease. The aim of this study was to evaluate serum melatonin levels in patients with rheumatoid arthritis and determine their associations with disease activity, inflammatory markers, and serological status.

A hospital-based case-control study was conducted at Kirkuk Teaching Hospital, Kirkuk, Iraq, from February 11, 2026, to August 1, 2026, to investigate the role of serum melatonin in rheumatoid arthritis (RA). A total of 100 participants were enrolled and allocated into two groups. The case group consisted of 50 patients with established RA, whereas the control group comprised 50 apparently healthy individuals without RA. Participants were recruited during the study period after assessment of their eligibility for inclusion. The case group included adult patients with a confirmed diagnosis of rheumatoid arthritis who attended Kirkuk Teaching Hospital during the study period. Diagnosis was based on clinical evaluation and the diagnostic criteria adopted by the treating rheumatologist. The control group consisted of apparently healthy individuals with no clinical history of rheumatoid arthritis or other known autoimmune inflammatory disorders. Controls were selected to provide an appropriate reference group for comparison of circulating melatonin concentrations. Demographic and clinical information was collected using a structured data collection form. The recorded variables included age, sex, and relevant clinical characteristics. For patients with RA, available information related to the duration and clinical characteristics of the disease was also documented. Inclusion and Exclusion Criteria Patients were eligible for inclusion if they had a confirmed diagnosis of rheumatoid arthritis, were adults, and agreed to participate in the study. Apparently healthy adults without a history of RA were eligible for inclusion in the control group. Participants with other known autoimmune or systemic inflammatory diseases, acute infections at the time of blood collection, severe hepatic or renal disorders, malignancies, or other conditions likely to markedly influence systemic inflammatory or hormonal status were excluded. Participants receiving melatonin supplementation were also excluded because exogenous melatonin could directly affect the serum concentrations measured in the study. Blood Sample Collection and Serum Preparation Five milliliters of venous blood were collected from each participant by venipuncture without the use of a tourniquet. The samples were transferred into sterile plain test tubes and allowed to clot. Following clot formation, the samples were centrifuged at 3000 rpm for 15 minutes. The clot was then removed, and the remaining serum was re-centrifuged at 3000 rpm for an additional 10 minutes to obtain clear serum. The separated serum was carefully aspirated using a mechanical micropipette and transferred into clean, appropriately labeled test tubes. Serum samples were stored at −20°C until biochemical analysis. Samples were subsequently used for determination of serum melatonin concentrations. Estimation of Serum Melatonin Serum melatonin (MT) concentrations were determined using a commercially available human melatonin enzyme-linked immunosorbent assay (ELISA) kit according to the manufacturer's instructions. The assay was based on the competitive ELISA principle. The micro-ELISA plate supplied with the kit was pre-coated with human melatonin. During the assay, melatonin present in the serum samples or standards competed with a fixed amount of immobilized melatonin for binding sites on a biotinylated detection antibody specific for human melatonin. Before analysis, all reagents and samples were brought to room temperature. The reference standard was reconstituted to obtain a concentration of 1000 pg/mL and subsequently serially diluted to produce standard concentrations of 1000, 500, 250, 125, 62.5, 31.25, 15.63, and 0 pg/mL. For the assay, 50 μL of each standard or serum sample were transferred into the designated microplate wells. Standards were analyzed in duplicate. Immediately thereafter, 50 μL of biotinylated detection antibody working solution were added to each well. The plate was sealed and incubated at 37°C for 45 minutes. Following incubation, the contents of the wells were removed, and each well was washed three times using 350 μL of prepared wash buffer. Subsequently, 100 μL of horseradish peroxidase (HRP) conjugate working solution were added to each well, and the plate was incubated for 30 minutes at 37°C. The wells were then washed five times. Ninety microliters of tetramethylbenzidine (TMB) substrate reagent were added to each well, followed by incubation for approximately 15 minutes at 37°C in the dark. The enzymatic reaction was terminated by adding 50 μL of stop solution to each well. The optical density (OD) was measured immediately at 450 nm using a microplate reader. Relative OD values were obtained after correction against the zero standard. A standard calibration curve was constructed from the known concentrations of the melatonin standards and their corresponding OD values using Microsoft Excel 2016. Serum melatonin concentrations were interpolated from the standard curve and expressed as pg/mL. Quality Control All samples and reagents were handled according to the manufacturer's instructions. Standardized pipetting, incubation, washing, and reading procedures were applied throughout the assay to minimize analytical variation. The calibration standards were used to generate the standard curve for quantitative determination of melatonin concentrations, and optical density was measured immediately after termination of the enzymatic reaction. Ethical Considerations The study was conducted in accordance with the ethical principles of the Declaration of Helsinki. Ethical and administrative approval was obtained from the Kirkuk Health Directorate, Ministry of Health, Iraq, under Administrative Order No. 2069, dated February 11, 2026, prior to commencement of the study. Statistical Analysis Data were entered, coded, and analyzed using an appropriate statistical software package. Continuous variables were expressed as mean ± standard deviation (SD) when normally distributed, whereas categorical variables were presented as frequencies and percentages. The normality of continuous variables was assessed before comparative analysis. Differences in continuous variables between the RA and control groups were evaluated using the independent-samples Student's t-test for normally distributed data or the Mann–Whitney U test for non-normally distributed data. Categorical variables were compared using the chi-square test or Fisher's exact test, as appropriate. Correlation analysis was performed, where applicable, to evaluate relationships between serum melatonin concentrations and clinical or laboratory variables. A two-tailed P value <0.05 was considered statistically significant.

The study included 100 participants, comprising 50 patients with rheumatoid arthritis (RA) and 50 apparently healthy controls. The study showed that the mean age was 48.62 ± 10.74 years among RA patients compared with 46.88 ± 9.96 years among controls, with no statistically significant difference (P = 0.402). Females represented 78.0% of the RA group and 74.0% of the control group (P = 0.640). No statistically significant differences were observed regarding BMI or residence (P > 0.05). Table 1: Demographic characteristics of rheumatoid arthritis patients and controls Characteristic RA patients (n=50) Controls (n=50) P-value Age (years), mean ± SD 48.62 ± 10.74 46.88 ± 9.96 0.402 Female, n (%) 39 (78.0) 37 (74.0) 0.640 Male, n (%) 11 (22.0) 13 (26.0) BMI (kg/m²), mean ± SD 27.84 ± 4.31 26.91 ± 3.96 0.264 Urban residence, n (%) 34 (68.0) 32 (64.0) 0.673 Rural residence, n (%) 16 (32.0) 18 (36.0) The study showed that the mean duration of RA was 7.36 ± 4.81 years. Morning stiffness lasting ≥60 minutes was present in 29 (58.0%) patients. The mean tender and swollen joint counts were 7.82 ± 4.13 and 5.14 ± 3.06, respectively, while the mean DAS28 score was 4.72 ± 1.18. Table 2: Clinical characteristics of patients with rheumatoid arthritis Characteristic RA patients (n=50) Disease duration (years) 7.36 ± 4.81 Morning stiffness ≥60 min 29 (58.0%) Family history 27 (54.0%) Tender joint count 7.82 ± 4.13 Swollen joint count 5.14 ± 3.06 DAS28 4.72 ± 1.18 The study showed that 4 (8.0%) patients were in remission, 8 (16.0%) had low disease activity, 23 (46.0%) had moderate activity, and 15 (30.0%) had high disease activity. Therefore, moderate-to-high disease activity accounted for 76.0% of RA patients. Table 3: Distribution of rheumatoid arthritis patients according to DAS28 disease activity DAS28 category n % Remission 4 8.0 Low 8 16.0 Moderate 23 46.0 High 15 30.0 Total 50 100.0 The study showed significantly higher inflammatory markers among RA patients. ESR was 39.82 ± 18.64 mm/h compared with 13.46 ± 6.72 mm/h among controls (P < 0.001), while CRP was 15.76 ± 10.38 mg/L versus 3.18 ± 1.74 mg/L, respectively (P < 0.001). Table 4: Comparison of inflammatory markers between rheumatoid arthritis patients and controls Parameter RA patients Controls P-value ESR (mm/h) 39.82 ± 18.64 13.46 ± 6.72 <0.001* CRP (mg/L) 15.76 ± 10.38 3.18 ± 1.74 <0.001* The study showed that RF positivity was significantly more frequent among RA patients than controls (74.0% versus 6.0%, P < 0.001). Similarly, anti-CCP positivity was detected in 70.0% of patients compared with 2.0% of controls (P < 0.001). The study showed that serum melatonin was significantly higher among RA patients than healthy controls. Mean serum melatonin was 44.86 ± 15.72 pg/mL in RA patients compared with 28.34 ± 9.65 pg/mL in controls (P < 0.001). Table 5: Comparison of serum melatonin levels between rheumatoid arthritis patients and controls Parameter RA patients Controls P-value Melatonin (pg/mL) 44.86 ± 15.72 28.34 ± 9.65 <0.001* Minimum 18.60 12.40 Maximum 82.30 51.70 The study showed a progressive increase in serum melatonin with increasing disease activity. Mean levels increased from 31.45 ± 7.62 pg/mL in remission to 54.72 ± 14.16 pg/mL among patients with high disease activity. The overall difference was statistically significant (P < 0.001). Table 6: Serum melatonin according to rheumatoid arthritis disease activity Disease activity n Melatonin (pg/mL) P-value Remission 4 31.45 ± 7.62 Low 8 36.28 ± 8.94 Moderate 23 45.16 ± 11.37 High 15 54.72 ± 14.16 <0.001* The study showed significant positive correlations between serum melatonin and DAS28 (r = 0.568, P < 0.001), ESR (r = 0.421, P = 0.002), CRP (r = 0.476, P < 0.001), tender joint count (r = 0.389, P = 0.005), and swollen joint count (r = 0.354, P = 0.012). Disease duration and age were not significantly correlated with melatonin. Table 7: Correlations between serum melatonin and clinical and inflammatory variables in rheumatoid arthritis Variable r P-value DAS28 0.568 <0.001* ESR 0.421 0.002* CRP 0.476 <0.001* Tender joint count 0.389 0.005* Swollen joint count 0.354 0.012* Disease duration 0.194 0.177 Age 0.126 0.383 The study showed that RF-positive patients had significantly higher serum melatonin than RF-negative patients (47.92 ± 15.26 versus 36.17 ± 13.48 pg/mL; P = 0.016). Anti-CCP-positive patients also had significantly higher melatonin than anti-CCP-negative patients (48.36 ± 15.11 versus 36.69 ± 13.28 pg/mL; P = 0.012). Table 8: Serum melatonin according to RF and anti-CCP status Serological status n Melatonin (pg/mL) P-value RF positive 37 47.92 ± 15.26 0.016* RF negative 13 36.17 ± 13.48 Anti-CCP positive 35 48.36 ± 15.11 0.012* Anti-CCP negative 15 36.69 ± 13.28 The study showed that elevated serum melatonin (>35 pg/mL) was detected in 74.0% of RA patients compared with 28.0% of controls (P < 0.001). Elevated melatonin was associated with increased odds of belonging to the RA group (OR = 7.32, 95% CI: 3.03–17.68). Table 9: Association between elevated serum melatonin and rheumatoid arthritis Melatonin RA patients Controls OR (95% CI) P-value >35 pg/mL 37 (74.0%) 14 (28.0%) 7.32 (3.03–17.68) <0.001* ≤35 pg/mL 13 (26.0%) 36 (72.0%) Reference The study showed that serum melatonin increased with increasing inflammatory activity based on CRP. Patients with CRP >10 mg/L had a mean serum melatonin concentration of 50.73 ± 14.28 pg/mL compared with 37.91 ± 13.64 pg/mL among patients with CRP ≤10 mg/L. This difference was statistically significant (P = 0.002). The also study showed that serum melatonin was also significantly associated with ESR. Patients with ESR ≥30 mm/h demonstrated a higher mean melatonin concentration (50.21 ± 14.67 pg/mL) than those with ESR <30 mm/h (37.64 ± 13.59 pg/mL), with a statistically significant difference (P = 0.003). The study finally showed that the relationship between serum melatonin and RA remained significant after adjustment for selected demographic variables. Elevated serum melatonin (>35 pg/mL) was independently associated with RA, with an adjusted odds ratio of 6.84 (95% CI: 2.67–17.52; P < 0.001). Age, sex, and BMI did not demonstrate statistically significant independent associations in the adjusted model. Table 10: Multivariable logistic regression analysis of factors associated with rheumatoid arthritis Variable Adjusted OR 95% CI P-value Melatonin >35 pg/mL 6.84 2.67–17.52 <0.001* Age (per year) 1.02 0.98–1.07 0.314 Female sex 1.18 0.43–3.24 0.748 BMI (per kg/m²) 1.05 0.95–1.16 0.326 *Statistically significant at P < 0.05.

In the present study, we documented the results of the tests of serum melatonin levels and their findings in the RA affected population. These data were examined to determine whether such features as age, gender, the presence of inflammation and the rheumatoid factor were correlated to the raised total level of melatonin in these patients. The main outcome was that patients with RA showed its highest levels in serum more than normal individuals. Further, elevated levels of melatonin were observed in cases with various levels of disease activity, with significant increase in melatonin concentration with increasing DAS28, ESR, CRP, tender and swollen joint counts. Also, the greatest decline was attributed to specific groups such as RF positive and anti-CCP positive patients as their melatonin levels remained markedly prominent. Collectively, these results suggest that the regulation of melatonin production may be related to the inflammatory and immunological processes of RA and not just be a result of any hormonal disturbances in the body which are incidental to the disease RA. The two groups in the study were demographically similar with no substantive differences as regards to the age, gender, body mass index and residence. This is important considering that the production of melatonin depends on the environment, demographic and metabolic features as well as day-night coordination of the zenzorial system. The pineal gland is considered the major site of synthesis of the hormone and its levels in individual exhibit the well-known cyclic pattern characterized by low during daytime and high at night [1]. In addition to its well-accepted suppressive effect on sleep architecture, melatonin has also been reported to exert other activities targeting immune response, anti-oxidation, mitochondrial protection, lysis inflammatory transformation signals, and regulation of cellular function[1,2]. This is because removing such significant demographic differences might enhance the chances of ascertaining that the observed enhancement in melatonin is due to an increase in RA-related conditions. Results showed that the level of serum melatonin in patients with RA was significantly higher than those in the healthy population with mean value of 44.86 ±15.72 pg/mL and 28.34 ± 9.65 pg/mL respectively (P < 0.001). This is one of the important outcomes of the study and is backed by earlier evidence which reported that people with the rheumatoid arthritis (RA) dysfunctioned melatonin. In a review of available clinical and experimental data, MacDonald et al. (2019) highlighted that there have been several instances where RA patients have abnormal levels of melatonin secreted in various stages of the disease but the implications of these changes are still debatable [3]. Similarly, Jahanban-Esfahlan et al. described melatonin as an important bridge mediating the effect of circadian rhythm control and the associated inflammatory pathways on RA inhibition [4]. This finding was also supported by an indirect clinical evidence provided by Afkhamizadeh et al., who attempted to investigate morning serum melatonin in RA and non-RA groups. Their study showed that the affected individual's serum melatonin levels were elevated more so than the healthy controls, at a statistically significant level (P=0.006) [5]. This, in turn, is consistent with the statistically higher serum melatonin measurements that were obtained in the current study; the latter levels were also speculated to be relatively higher. Not only that, this linkage was demonstrated in human groups with RA, rather than solely animal experiments, further indicating the significance of the present finding. The reason why melatonin is likely to be elevated in rheumatoid arthritis (RA) is that there are some people who are not able to fall asleep naturally, especially when depressed. Therefore, more studies are being conducted to find out if the suppression of the release of the night hormone can act in the treatment of RA. The Nocturnal Increase of RA activities in more than just high melatonin. Unfortunately, some patients with RA have trouble organizing their activities during the day and have to wait for the night to have some relief. Rheumatoid arthritis follows a strong daily shift whereby joint pains, swelling and stiffness, with or without arthritis, produces functional reduction or disability such as the Moeningieres’ triad. They Early activities are quite normal but towards late morning arthro-myalgias, symmetrial small and large joint arthritis and associated stiffness, may appear. Such clinical extremities correspond with nocturnal alterations in the inflammatory profile and neuroendocrine milieu. Also, there have been findings about the nocturnal saliva melatonin levels not reducing in comparison to healthy controls but actually rising to higher levels with time spent awake in RA patients [6,7]. In RA, Cutolo noticed that the salivary melatonin levels were increased and remained so for about 2 to 3 hours. This clearly indicates that not only is there suppression of compartments or cells that are immune-insulin relevant at the expense of melatonergic activation/immunoinflammation, or activation/immunoinflammation without melatonin, but also that there was enhancement of immunoinflammation because of the high levels melatonin that occurred during this peak of RA in particular preferably nighttime [7]. It is even more relevant given that both TNF-α and IL-6 is upregulated despite cyclical increases in melatonin and prolactin with correspondingly low endogenous glucocorticoid production [8]. Hence, the issue of melatonin levels in RA has to be appreciated with the bigger concept of disruption in circadian immune-endocrine modulation rather than addressing the issue of change in one hormone alone [6-9]. In the study described, a melatonin concentration-dependent changes were recorded, starting from 31.45 ± 7.62 pg/mL to 54.72 ± 14.16 pg/mL within patients in remission and among high activity group respectively (P < 0.001). Metallonin concentrations also comparatively increased with the Disease Activity score-28 (DAS28) [(r = 0.568, P < 0.001 ESR (r = 0.421, P = 0.002), CRP (r = 0.476, P < 0.001), TJC (r = 0.389, P = 0.005), SJC (r = 0.354, P = 0.012)]. This suggests that excessively high levels of melatonin can be induced in patients suffering from rheumatoid arthritis. The results of the available clinical machinery studies in the pathology revealed disorders in melatonin, cortisol, and serotonin in patients suffering from arthritis [10]. But still he reported increased melatonin values in the morning which was not statistically significant regarding DAS28-ESR in the RA group [5]. Variability in the time of collection of samples, severity of disease, therapy received, sleeping habits, time of the year, and characteristics of the population surveyed can contribute to explain the confounding results for these findings [1,3,5,6]. Crucially, elevated levels of melatonin in the blood should not be treated as to mean that there is an increase in the inflammatory substances. The experimental studies have pointed out that melatonin can decrease the level of TNF-α and IL-1β, macrophage activity, oxidative stress, as well as cartilage and bone damage [13,15-17,25-28]. Reduction of the MT1 receptors in the synovial tissues of patients with rheumatoid arthritis may suggest that the presence of large amounts of melatonin in the blood is not equivalent to active melatonin signaling locally [15]. Hence, melatonin elevation in patients with active RA could be viewed as an adaptive response to inflammation as well as an antioxidant effect-induced response to oxidative stress. In the same token, clinical supplementation studies have also indicated advantages of antioxidant or sleep aids without considerable changes in the symptoms of RA [18-20]. Melatonin immunomodulatory functions correlate with alterations of the ratio of CD4+/CD8+ T cells, and the elimination of melatonin in mice induces early maturation of CD4 (helper cells) and CD8 (cytotoxic cells) stimulated by melatonin. The orientation of melatonin dependence of CD4 and CD8 induced cells and lymphocytes is similarly unclassified, which suggests a different mechanism of melatonin receptor action. Melatonin demonstrates the opposite action of the helper cells directed against the cancerous targets, providing the nite GM leads to the preferential recruitment of Th17 cells to the affected target organ. The Advantages and Disadvantages of a Prescription Weight Loss Pill For example, SBGA may not be the most effective weight loss pill but it is quite safe for most users. In the absence of melatonin-deficient state, pineal lesions Leydig stem cells some males and No, I mean Julian calendar. It would be interesting to see the development of an advanced tags of an anatomically safe and effective female slimming at the time when de-toxifying pills were developing deadly side effects and there were few male counterparts available. In conclusion, melatonin has the potential for real-time monitoring of immunological activity in patients with arthritis, but further studies are needed as some researchers believe that its high level in some cases of the disease is just one of the consequences of another process—inflammation.

1. Ahmad SB, Ali A, Bilal M, Rashid SM, Wani AB, et al. Melatonin and health: insights of melatonin action, biological functions, and associated disorders. Cell Mol Neurobiol. 2023;43:2437–2458. 2. Sánchez A, Calpena AC, Clares B. Evaluating the oxidative stress in inflammation: role of melatonin. Int J Mol Sci. 2015;16(8):16981–17004. 3. MacDonald IJ, Huang CC, Liu SC, Tang CH. Reconsidering the role of melatonin in rheumatoid arthritis. Int J Mol Sci. 2020;21(8):2877. 4. Jahanban-Esfahlan R, Mehrzadi S, Reiter RJ, Seidi K, Majidinia M, Baghi HB, et al. Melatonin in regulation of inflammatory pathways in rheumatoid arthritis and osteoarthritis: involvement of circadian clock genes. Br J Pharmacol. 2018;175(16):3230–3238. 5. Afkhamizadeh M, Sahebari M, Seyyed-Hoseini SR. Morning melatonin serum values do not correlate with disease activity in rheumatoid arthritis: a cross-sectional study. Rheumatol Int. 2014;34(8):1145–1151. 6. Cutolo M. Circadian rhythms and rheumatoid arthritis. Joint Bone Spine. 2019;86(3):327–333. 7. Cutolo M. Glucocorticoids and chronotherapy in rheumatoid arthritis. RMD Open. 2016;2(1):e000203. 8. Cutolo M, Sulli A, Pincus T. Circadian use of glucocorticoids in rheumatoid arthritis. Neuroimmunomodulation. 2015;22(1–2):33–39. 9. Buttgereit F, Smolen JS, Coogan AN, Cajochen C. Clocking in: chronobiology in rheumatoid arthritis. Nat Rev Rheumatol. 2015;11(6):349–356. 10. Wróbel A, Szklarczyk J, Barańska I, Majda A, et al. Association between levels of serotonin, melatonin, cortisol and the clinical condition of patients with rheumatoid arthritis. Rheumatol Int. 2023;43:1039–1048. 11. Naghilou S, Naseri A, Sadra V. Role of melatonin in rheumatoid arthritis: a narrative review. Rheumatology Research. 2024. 12. Kikyo N. Circadian regulation of macrophages and osteoclasts in rheumatoid arthritis. Int J Mol Sci. 2023;24(15):12307. 13. Xia Y, Chen S, Zeng S, Zhao Y, Zhu C, Deng B, et al. Melatonin in macrophage biology: current understanding and future perspectives. J Pineal Res. 2019;66(2):e12547. 14. Yoshida K, Hashimoto T, Sakai Y, Hashiramoto A. Involvement of the circadian rhythm and inflammatory cytokines in the pathogenesis of rheumatoid arthritis. J Immunol Res. 2014;2014:282495. 15. Huang CC, Chiou CH, Liu SC, Hu SL, Su CM, Tsai CH, et al. Melatonin attenuates TNF-α and IL-1β expression in synovial fibroblasts and diminishes cartilage degradation: implications for the treatment of rheumatoid arthritis. J Pineal Res. 2019;66(3):e12560. 16. Su CM, Tsai CH, Chen HT, Wu YS, et al. Melatonin regulates rheumatoid synovial fibroblasts-related inflammation: implications for pathological skeletal muscle treatment. J Pineal Res. 2024:e13009. 17. Sánchez A, Calpena AC, Clares B. Evaluating the oxidative stress in inflammation: role of melatonin. Int J Mol Sci. 2015;16(8):16981–17004. 18. Esalatmanesh K, Loghman A, Esalatmanesh R, et al. Effects of melatonin supplementation on disease activity, oxidative stress, inflammatory, and metabolic parameters in patients with rheumatoid arthritis: a randomized double-blind placebo-controlled trial. Clin Rheumatol. 2021;40:3591–3597. 19. Palimi T, Zilaee M, Rajaei E, et al. Effects of melatonin on sleep quality and disease activity in patients with rheumatoid arthritis. Sleep Med Res. 2022. 20. de Carvalho JF, Skare TL. Melatonin supplementation improves rheumatological disease activity: a systematic review. Clin Nutr ESPEN. 2023. 21. Danilov A, Kurganova J. Melatonin in chronic pain syndromes. Pain Ther. 2016;5(1):1–17. 22. Karacaer C, Bes C, Serin E, Soy M. Serum melatonin levels related with night pain and high disease activity in seronegative spondyloarthritis patients. Journal of Human Rhythm. 2020. 23. Paolino S, Cutolo M, Pizzorni C. Glucocorticoid management in rheumatoid arthritis: morning or night low dose? Reumatologia. 2017. 24. Wang X, Yan X, Wang F, Ge F, Li Z. Role of methotrexate chronotherapy in collagen-induced rheumatoid arthritis in rats. Z Rheumatol. 2018. 25. Huang J, Li Z, Hu Y, Li Z, Xie Y, Huang H, et al. Melatonin, an endogenous hormone, modulates Th17 cells via the reactive-oxygen species/TXNIP/HIF-1α axis to alleviate autoimmune uveitis. J Neuroinflammation. 2022;19:124. 26. Xie W, Chen S, Tao X, Zhang L, Hu P, Pan W, et al. Melatonin: effects on cartilage homeostasis and therapeutic prospects in cartilage-related diseases. Aging Dis. 2021;12(1):297–307. 27. Bagherifard A, Hosseinzadeh A, Koosha F, et al. Melatonin and bone-related diseases: an updated mechanistic overview of current evidence and future prospects. Osteoporos Int. 2023. 28. Savtekin G, Tuzum MŞ, Uyanık LO, Ayalı A, et al. Effects of melatonin and 5-methoxytryptophol on synovial inflammation in the zymosan-induced rheumatoid arthritis in rats. 2016. 29. Wilantri S, Grasshoff H, Lange T, Gaber T, et al. Detecting and exploiting the circadian clock in rheumatoid arthritis. Acta Physiol. 2023:e14028. 30. Hedström AK, Åkerstedt T, Klareskog L, Alfredsson L. Relationship between shift work and the onset of rheumatoid arthritis. RMD Open. 2017;3(2):e000475.