The Impact of Melatonin in Research.

Citation indexes represent helpful tools for evaluating the impact of articles on research. The aim of this study was to obtain the top-100 ranking of the most cited papers on melatonin, a relevant neurohormone mainly involved in phase-adjusting the biological clock and with certain sleep-promoting capability. An article search was carried out on the Institute for Scientific Information (ISI) Web of Science platform. Numbers of citations, names of authors, journals and their 2014-impact factor, year of publication, and experimental designs of studies were recorded. The ranking of the 100-most cited articles on melatonin research (up to February 2016) revealed a citation range from 1623 to 310. Narrative reviews/expert opinions were the most frequently cited articles, while the main research topics were oxidative stress, sleep physiology, reproduction, circadian rhythms and melatonin receptors. This study represents the first detailed analysis of the 100 top-cited articles published in the field of melatonin research, showing its impact and relevance in the biomedical field.

1. Introduction

By simply typing the word “melatonin” on the PubMed database, more than 1000 records can be easily retrieved, for just year 2015. Melatonin, the main sleep-promoting neurohormone involved in phase-adjusting the circadian clockworks upon prior phase-shifting, recently received overpowering attention in science, medicine and social media, and it is expected to gain even more attention within the near future.

Such a feeling is strongly supported by analyzing the “citation index” of this molecule, i.e. how many times researchers have cited papers on melatonin over time. “Citation index” is, to date, one of the most reliable methods for assessing the quality and the “scientific power” of a paper, a journal or an issue [1], reflecting its impact on research, opening further discussion, producing changes in clinical practice, starting controversy inside scientific community and providing new perspectives in science and in financial funding as well.

The top-100 rank of the highly cited papers provides an interesting picture of the current “hot” topics, even delineating those trends expected to further explode in the future. Along this direction, the rankings of the 100 top-cited articles have been published in a plethora of biomedical disciplines, such as emergency medicine [2], cardiology [3], orthopedic surgery [4] and dentistry [5]. Considering the wide and increasing interest on this hormone, this work aims to provide and analyze the ranking of the 100 top-cited articles on melatonin research.

2. Results

During our search we excluded only one article for being out-of-topic, namely a review by Del Rio et al. on the toxic molecule malondialdehyde as a biological marker of oxidative stress [6]. We found that the number of citations in the top-100 rank (Table 1), ranged between 1623 and 310; each of the first five articles exceeded 1000 citations and the first sixty articles had more than 400 citations. These findings provide the major, pivotal hint of the huge impact of melatonin in science, since all papers of our ranking had more than 100 citations, the latter considered the threshold to identify a “classic” article [7,8]—the “last” article, at position 100, had 310 citations. This also suggests that due to the nature of this ranking, a very large number of classics have not been here included, despite their undeniable scientific importance.

Table 1

Top-100 rank of the most cited articles in the field of melatonin research.

Ranking	Article	Citations
1	Valko, M.; Morris, H.; Cronin, M.T.D. Metals, toxicity and oxidative stress. Curr. Med. Chem. 2005, 12, 1161–1208 [9].	1623
2	Reiter, R.J. Pineal melatonin: Cell biology of its synthesis and of its physiological interactions. Endocr. Rev. 1991, 12, 151–180 [10].	1572
3	Tan, D.-X.; Chen, L.D.; Poeggeler, B.; Manchester, L.C.; Reiter, R.J.; others. Melatonin: A potent, endogenous hydroxyl radical scavenger. Endocr. J. 1993, 1, 57–60 [11].	1420
4	Reiter, R.J. The pineal and its hormones in the control of reproduction in mammals. Endocr. Rev. 1980, 1, 109–131 [12].	1219
5	Lewy, A.J.; Wehr, T.A.; Goodwin, F.K.; Newsome, D.A.; Markey, S.P. Light suppresses melatonin secretion in humans. Science 1980, 210, 1267–1269 [13].	1105
6	Maritim, A.C.; Sanders, R.A.; Watkins, J.B. Diabetes, oxidative stress, and antioxidants: A review. J. Biochem. Mol. Toxicol. 2003, 17, 24–38 [14].	975
7	Rodriguez, C.; Mayo, J.C.; Sainz, R.M.; Antolín, I.; Herrera, F.; Martín, V.; Reiter, R.J. Regulation of antioxidant enzymes: A significant role for melatonin. J. Pineal Res. 2004, 36, 1–9 [15].	886
8	Lerner, A.B.; Case, J.D.; Takahashi, Y.; Lee, T.H.; Mori, W. Isolation of melatonin, the pineal gland factor that lightens melanocytes. J. Am. Chem. Soc. 1958, 80, 2587–2587 [16].	836
9	Axelrod, J. The pineal gland: A neurochemical transducer. Science 1974, 184, 1341–1348 [17].	815
10	Brzezinski, A. Melatonin in humans. N. Engl. J. Med. 1997, 336, 186–195 [18].	802
11	Reppert, S.M.; Weaver, D.R.; Ebisawa, T. Cloning and characterization of a mammalian melatonin receptor that mediates reproductive and circadian responses. Neuron 1994, 13, 1177–1185 [19].	802
12	Klein, D.C.; Weller, J.L. Indole metabolism in the pineal gland: A circadian rhythm in N-acetyltransferase. Science 1970, 169, 1093–1095 [20].	757
13	Ancoli-Israel, S.; Cole, R.; Alessi, C.; Chambers, M.; Moorcroft, W.; Pollak, C.P. The role of actigraphy in the study of sleep and circadian rhythms. Sleep 2003, 26, 342–392 [21]	768
14	Tan, D.-X.; Manchester, L.C.; Terron, M.P.; Flores, L.J.; Reiter, R.J. One molecule, many derivatives: A never-ending interaction of melatonin with reactive oxygen and nitrogen species? J. Pineal Res. 2007, 42, 28–42 [22].	765
15	Toh, K.L.; Jones, C.R.; He, Y.; Eide, E.J.; Hinz, W.A.; Virshup, D.M.; Ptácek, L.J.; Fu, Y.H. An hPer2 phosphorylation site mutation in familial advanced sleep phase syndrome. Science 2001, 291, 1040–1043 [23].	716
16	Rollag, M.D.; Niswender, G.D. Radioimmunoassay of serum concentrations of melatonin in sheep exposed to different lighting regimens. Endocrinology 1976, 98, 482–489 [24].	688
17	Reiter, R.J. Oxidative processes and antioxidative defense mechanisms in the aging brain. FASEB J 1995, 9, 526–533 [25].	678
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40	Lewy, A.J.; Ahmed, S.; Jackson, J.M.; Sack, R.L. Melatonin shifts human circadian rhythms according to a phase-response curve. Chronobiol. Int. 1992, 9, 380–392 [48]	464
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42	Liu, C.; Weaver, D.R.; Jin, X.; Shearman, L.P.; Pieschl, R.L.; Gribkoff, V.K.; Reppert, S.M. Molecular dissection of two distinct actions of melatonin on the suprachiasmatic circadian clock. Neuron 1997, 19, 91–102 [50].	446
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48	Goldman, B.D. Mammalian photoperiodic system: Formal properties and neuroendocrine mechanisms of photoperiodic time measurement. J. Biol. Rhythms 2001, 16, 283–301 [56].	425
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53	Reiter, R.J.; Tan, D.X.; Manchester, L.C.; Qi, W. Biochemical reactivity of melatonin with reactive oxygen and nitrogen species: a review of the evidence. Cell Biochem. Biophys. 2001, 34, 237–256 [61].	418
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56	Reppert, S.M.; Weaver, D.R.; Rivkees, S.A.; Stopa, E.G. Putative melatonin receptors in a human biological clock. Science 1988, 242, 78–81 [64].	413
57	Kamberi, I.A.; Mical, R.S.; Porter, J.C. Effects of melatonin and serotonin on the release of FSH and prolactin. Endocrinology 1971, 88, 1288–1293 [65].	409
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60	Dawson, A.; King, V.M.; Bentley, G.E.; Ball, G.F. Photoperiodic control of seasonality in birds. J. Biol. Rhythms 2001, 16, 365–380 [68].	401
61	Dubocovich, M.L. Melatonin is a potent modulator of dopamine release in the retina. Nature 1983, 306, 782–784 [69].	399
62	Wurtman, R.J.; Axelrod, J.; Phillips, L.S. Melatonin synthesis in the pineal gland: control by light. Science 1963, 142, 1071–1073 [70].	392
63	Bartness, T.J.; Powers, J.B.; Hastings, M.H.; Bittman, E.L.; Goldman, B.D. The timed infusion paradigm for melatonin delivery: What has it taught us about the melatonin signal, its reception, and the photoperiodic control of seasonal responses? J. Pineal Res. 1993, 15, 161–190 [71].	394
64	Klein, D.C.; Coon, S.L.; Roseboom, P.H.; Weller, J.L.; Bernard, M.; Gastel, J.A.; Zatz, M.; Iuvone, P.M.; Rodriguez, I.R.; Bégay, V.; et al. The melatonin rhythm-generating enzyme: molecular regulation of serotonin N-acetyltransferase in the pineal gland. Recent Prog. Horm. Res. 1997, 52, 307–357; discussion 357–358 [72].	391
65	Vanecek, J. Cellular mechanisms of melatonin action. Physiol. Rev. 1998, 78, 687–721 [73]	389
66	Carter, D.S.; Goldman, B.D. Antigonadal effects of timed melatonin infusion in pinealectomized male Djungarian hamsters (Phodopus sungorus sungorus): Duration is the critical parameter. Endocrinology 1983, 113, 1261–1267 [74].	385
67	Zeitzer, J.M.; Dijk, D.J.; Kronauer, R.; Brown, E.; Czeisler, C. Sensitivity of the human circadian pacemaker to nocturnal light: Melatonin phase resetting and suppression. J. Physiol. 2000, 526 (Pt 3), 695–702 [75].	385
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69	Hill, S.M.; Blask, D.E. Effects of the pineal hormone melatonin on the proliferation and morphological characteristics of human breast cancer cells (MCF-7) in culture. Cancer Res. 1988, 48, 6121–6126 [77].	363
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71	Pandi-Perumal, S.R.; Srinivasan, V.; Maestroni, G.J. M.; Cardinali, D.P.; Poeggeler, B.; Hardeland, R. Melatonin: Nature’s most versatile biological signal? FEBS J. 2006, 273, 2813–2838 [79].	376
72	Galano, A.; Tan, D.X.; Reiter, R.J. Melatonin as a natural ally against oxidative stress: A physicochemical examination. J. Pineal Res. 2011, 51, 1–16 [80].	399
73	Bromage, N.; Porter, M.; Randall, C. The environmental regulation of maturation in farmed finfish with special reference to the role of photoperiod and melatonin. Aquaculture 2001, 197, 63–98 [81].	368
74	Poeggeler, B.; Reiter, R.J.; Tan, D.X.; Chen, L.D.; Manchester, L.C. Melatonin, hydroxyl radical-mediated oxidative damage, and aging: a hypothesis. J. Pineal Res. 1993, 14, 151–168 [82].	362
75	Jezek, P.; Hlavatá, L. Mitochondria in homeostasis of reactive oxygen species in cell, tissues, and organism. Int. J. Biochem. Cell Biol. 2005, 37, 2478–2503 [83].	356
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77	Stehle, J.H.; Foulkes, N.S.; Molina, C.A.; Simonneaux, V.; Pévet, P.; Sassone-Corsi, P. Adrenergic signals direct rhythmic expression of transcriptional repressor CREM in the pineal gland. Nature 1993, 365, 314–320 [85].	353
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80	Cassone, V.M. Effects of melatonin on vertebrate circadian systems. Trends Neurosci. 1990, 13, 457–464 [88].	346
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82	Czeisler, C.A.; Shanahan, T.L.; Klerman, E.B.; Martens, H.; Brotman, D.J.; Emens, J.S.; Klein, T.; Rizzo, J.F. Suppression of melatonin secretion in some blind patients by exposure to bright light. N. Engl. J. Med. 1995, 332, 6–11 [90].	345
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The first paper, with 1623 citations, presented a narrative review which included melatonin in the response to oxidative [9]. This was also the first article in the ranking based on the annual citation rate (ACR), i.e., the ratio between the number of citations of a paper and the number of years since its publication: Valko et al. collected 1490 citations in 10 years with an ACR of 162.3 (Table 2). The ACR classification also highlighted the work by Galano and colleagues [80] which recorded a very high ACR (=99.7), because their paper, just published in 2011, collected 356 citations.

Table 2

Top 10 articles according to their annual citation rate (ACR, citations/year).

Ranking	Article	ACR
1	Valko, M.; Morris, H.; Cronin, M.T. D. Metals, toxicity and oxidative stress. Curr. Med. Chem. 2005, 12, 1161–1208 [9].	162.3
2	Galano, A.; Tan, D.X.; Reiter, R.J. Melatonin as a natural ally against oxidative stress: A physicochemical examination. J. Pineal Res. 2011, 51, 1–16 [80].	99.7
3	Tan, D.-X.; Manchester, L.C.; Terron, M.P.; Flores, L.J.; Reiter, R.J. One molecule, many derivatives: A never-ending interaction of melatonin with reactive oxygen and nitrogen species? J. Pineal Res. 2007, 42, 28–42 [22].	95.6
4	Maritim, A.C.; Sanders, R.A.; Watkins, J.B. Diabetes, oxidative stress, and antioxidants: A review. J. Biochem. Mol. Toxicol. 2003, 17, 24–38 [14].	81.25
5	Rodriguez, C.; Mayo, J.C.; Sainz, R.M.; Antolín, I.; Herrera, F.; Martín, V.; Reiter, R.J. Regulation of antioxidant enzymes: A significant role for melatonin. J. Pineal Res. 2004, 36, 1–9 [15].	80.5
6	Reiter, R.J. Pineal melatonin: Cell biology of its synthesis and of its physiological interactions. Endocr. Rev. 1991, 12, 151–180 [10].	65.5
7	Tan, D.-X.; Chen, L.D.; Poeggeler, B.; Manchester, L.C.; Reiter, R.J.; others. Melatonin: A potent, endogenous hydroxyl radical scavenger. Endocr. J. 1993, 1, 57–60 [11].	64.5
8	Ancoli-Israel, S.; Cole, R.; Alessi, C.; Chambers, M.; Moorcroft, W.; Pollak, C.P. The role of actigraphy in the study of sleep and circadian rhythms. Sleep 2003, 26, 342–392 [21].	64.0
9	Prokopenko, I.; Langenberg, C.; Florez, J.C.; Saxena, R.; Soranzo, N.; Thorleifsson, G.; Loos, R.J.F.; Manning, A.K.; Jackson, A.U.; Aulchenko, Y.; Potter, S.C.; et al. Variants in MTNR1B influence fasting glucose levels. Nat. Genet. 2009, 41, 77–81 [89].	57.6
10	Toh, K.L.; Jones, C.R.; He, Y.; Eide, E.J.; Hinz, W.A.; Virshup, D.M.; Ptácek, L.J.; Fu, Y.H. An hPer2 phosphorylation site mutation in familial advanced sleep phase syndrome. Science 2001, 291, 1040–1043 [23].	51.1

In second place of the top 100 list rank one could find another narrative review by Reiter and colleagues, published in 1991 [10]. The work entitled “Melatonin: a potent, endogenous hydroxyl radical scavenger”, with 1420 citations, completed the podium [109]: This was an original article, published by Tan et al., in 1993. The fourth paper , with 1,219 citations [109] was a narrative review published by Reiter et al., in 1980. The top 5 ranking concluded with a paper by Lewy and colleagues published in Science in 1980, with 1105 citations [13].

Unexpectedly, scientific works with limited evidence were cited the most. For the most part, indeed, articles were narrative reviews/expert opinions (33%), followed by basic research/descriptive studies (25%), whilst the less represented papers were systematic reviews (23%) and clinical trials (19%) (Figure 1).

Figure 1

Methodological designs of the top 100 most cited papers in melatonin research.

Because reviews are usually more frequently cited, two different top 10 rankings were created to minimize this bias, in order to evaluate in details the number of citations for reviews vs. original articles (Table 3 and Table 4). We included one letter to the editor among the original articles [35], since it reported a novel non-extraction radioimmunoassay (RIA) to detect melatonin in plasma. Interestingly, in both the classifications, melatonin as antioxidant agent and its role in physiology, mainly in regulating mammal reproduction, were the most cited topics.

Table 3

The top 10 most cited reviews.

Ranking	Article	Citations
1	Valko, M.; Morris, H.; Cronin, M.T. D. Metals, toxicity and oxidative stress. Curr. Med. Chem. 2005, 12, 1161–1208 [9].	1623
2	Reiter, R.J. Pineal melatonin: Cell biology of its synthesis and of its physiological interactions. Endocr. Rev. 1991, 12, 151–180 [10].	1572
3	Reiter, R.J. The pineal and its hormones in the control of reproduction in mammals. Endocr. Rev. 1980, 1, 109–131 [12].	1219
4	Maritim, A.C.; Sanders, R.A.; Watkins, J.B. Diabetes, oxidative stress, and antioxidants: A review. J. Biochem. Mol. Toxicol. 2003, 17, 24–38 [14].	975
5	Rodriguez, C.; Mayo, J.C.; Sainz, R.M.; Antolín, I.; Herrera, F.; Martín, V.; Reiter, R.J. Regulation of antioxidant enzymes: A significant role for melatonin. J. Pineal Res. 2004, 36, 1–9 [15].	886
6	Lerner A.B.; Case, J.D.; Takahashi, Y.; Lee, T.; Mori, W. Isolation of melatonin, the pineal gland factor that lightens melanocytes. J. Am. Chem. Soc. 1958, 80, 2587–2587 [16].	836
7	Axelrod, J. The pineal gland: A neurochemical transducer. Science 1974, 184, 1341–1348 [17].	815
8	Brzezinski, A. Melatonin in humans. N. Engl. J. Med. 1997, 336, 186–195 [18].	802
9	Ancoli-Israel, S.; Cole, R.; Alessi, C.; Chambers, M.; Moorcroft, W.; Pollak, C.P. The role of actigraphy in the study of sleep and circadian rhythms. Sleep 2003, 26, 342–392 [21].	768
10	Klein, D.C.; Weller, J.L. Indole metabolism in the pineal gland: A circadian rhythm in N-acetyltransferase. Science 1970, 169, 1093–1095 [20].	757

Table 4

The top 10 most cited original articles.

Ranking	Article	Citations
1	Tan, D.-X.; Chen, L.D.; Poeggeler, B.; Manchester, L.C.; Reiter, R.J.; others. Melatonin: A potent, endogenous hydroxyl radical scavenger. Endocr J 1993, 1, 57–60 [11].	1420
2	Lewy, A.J.; Wehr, T.A.; Goodwin, F.K.; Newsome, D.A.; Markey, S.P. Light suppresses melatonin secretion in humans. Science 1980, 210, 1267–1269 [13].	1105
3	Reppert, S.M.; Weaver, D.R.; Ebisawa, T. Cloning and characterization of a mammalian melatonin receptor that mediates reproductive and circadian responses. Neuron 1994, 13, 1177–1185 [19].	802
4	Toh, K.L.; Jones, C.R.; He, Y.; Eide, E.J.; Hinz, W.A.; Virshup, D.M.; Ptácek, L.J.; Fu, Y.H. An hPer2 phosphorylation site mutation in familial advanced sleep phase syndrome. Science 2001, 291, 1040–1043 [23].	716
5	Rollag, M.D.; Niswender, G.D. Radioimmunoassay of serum concentrations of melatonin in sheep exposed to different lighting regimens. Endocrinology 1976, 98, 482–489 [24].	688
6	Reppert, S.M.; Godson, C.; Mahle, C.D.; Weaver, D.R.; Slaugenhaupt, S.A.; Gusella, J.F. Molecular characterization of a second melatonin receptor expressed in human retina and brain: The Mel1b melatonin receptor. Proc. Natl. Acad. Sci. USA 1995, 92, 8734–8738 [30].	641
7	Fraser, S.; Cowen, P.; Franklin, M.; Franey, C.; Arendt, J. Direct radioimmunoassay for melatonin in plasma. Clin. Chem. 1983, 29, 396–397 [35].	554
8	Cao, G.; Prior, R.L. Comparison of different analytical methods for assessing total antioxidant capacity of human serum. Clin. Chem. 1998, 44, 1309–1315 [38].	516
9	Provencio, I.; Rodriguez, I.R.; Jiang, G.; Hayes, W.P.; Moreira, E.F.; Rollag, M.D. A novel human opsin in the inner retina. J. Neurosci. Off. J. Soc. Neurosci. 2000, 20, 600–605 [39].	514
10	Schernhammer, E.S.; Laden, F.; Speizer, F.E.; Willett, W.C.; Hunter, D.J.; Kawachi, I.; Colditz, G.A. Rotating night shifts and risk of breast cancer in women participating in the nurses’ health study. J. Natl. Cancer Inst. 2001, 93, 1563–1568 [40].	514

The golden age for melatonin research, accounting for the largest number of “most-cited” publications, was the 1990–1999 decade, with 35 articles (Figure 2a). This decade also showed the highest number of total citations (18,604, Figure 2b). The 2000s followed with 31 papers and 16,182 total citations. The highest mean of the number of citations, calculated as the total citations from the total number of top-100 articles per decade, was, instead, recorded for the decade of the 1950s, with 578 mean citations (Figure 2b).

Figure 2

Years of publication (a) of the top 100 most cited articles in the field of melatonin research. Number of citations (b) and mean of the number of citations per decade (c).

The top 100 most cited articles were published in 52 different journals (Table 5). The journal with the largest number of papers was Science, with 13 articles, four of them within the first 20. It was followed by the Journal of Pineal Research and Endocrinology, with eight and five papers, respectively.

Table 5

Ranking of the journals with articles within the top 100 list.

Ranking	Journal	No Articles
1	Science (33.611)	13
2	Journal of Pineal Research (9.600)	8
3	Endocrinology (4.503)	5
4	Proceedings of the National Academy of Sciences USA (9.674)	4
5	Endocrine Reviews (21.059)	3
6	Journal of Neuroscience (6.344)	3
7	Journal of the National Cancer Institute (12.583)	3
8	Nature (41.456)	3
9	Recent Progress in Hormone Research	3
10	Clinical Chemistry (7.911)	2
11	FASEB Journal (5.043)	2
12	Journal of Biological Rhythms (4.573)	2
13	Journal of Physiology - London (5.037)	2
14	Journal of the American Chemical Society (12.113)	2
15	Life Sciences (2.702)	2
16	Neurochemistry International (3.092)	2
17	Neuron (15.054)	2
18	Physiological Reviews (27.324)	2
19	The Journal of Biological Chemistry (4.573)	2
20	The New England Journal of Medicine (55.873)	2
21	Trends in Pharmacological Sciences (11.539)	2
22	Aquaculture (1.878)	1
23	Alternative Medicine Review (3.833)	1
24	Biochemical Pharmacology (5.009)	1
25	Biological Signals and Receptors (2.000)	1
26	Brain Research (2.843)	1
27	Cancer Research (9.329)	1
28	Cell (32.242)	1
29	Cell Biochemistry and Biophysics (1.680)	1
30	Chronobiology International (3.343)	1
31	Current Medicinal Chemistry (3.853)	1
32	Current Topics in Medicinal Chemistry (3.402)	1
33	Digestive Diseases and Sciences (2.613)	1
34	Endocrine Journal (1.997)	1
35	Experientia (5.808)	1
36	FEBS Journal (4.001)	1
37	International Journal of Biochemistry and Cell Biology (4.046)	1
38	Journal of Biochemistry and Molecular Toxicology (1.925)	1
39	Journal of Biomedical Science (2.763)	1
40	Journal of Clinical Psychiatry (5.498)	1
41	Journal of Comparative Physiology (2.036)	1
42	Journal of Neuroimmunology (2.467)	1
43	Nature Genetics (29.352)	1
44	Pharmacological Reviews (17.099)	1
45	Progress in Neurobiology (9.992)	1
46	Quarterly Review of Biology (4.889)	1
47	Sleep (4.591)	1
48	The Journal of Cell Biology (9.834)	1
49	The Lancet (45.217)	1
50	Toxicology (3.621)	1
51	Trends in Immunology (10.399)	1
52	Trends in Neurosciences (13.555)	1
	Total	100

Surprisingly, no correlation could be observed between the number of citations in this ranking and the impact factors of the journals where papers were published (linear regression: R2 = 0.0021, Figure 3).

Figure 3

Linear correlation between number of citations of articles included in the top 100 list and impact factors of journals where papers were published.

The authors with the highest number of articles within the rank were Reiter with 16 papers (first author in nine of them), followed by Tan with nine papers (four as first author) and Reppert with six articles (five as first author) (Table 6). At fourth place, Weaver had five papers and was first Author in one of them. At fifth place, Axelrod, Nobel Prize in Physiology or Medicine in 1970, had five article and he was first author in two of them. Axelrod was ex equo with Manchester.

Table 6

Top five authors with the most cited papers on melatonin research.

Name	First Author	Co-Author	Last Author	Total
1. Reiter, RJ	9	4	3	16
2. Tan, D-X	4	3	2	9
3. Reppert, SM	4		1	5
4. Weaver, DR	1	4	0	5
5. Axelrod, J	2	2	1	5
6. Manchester, LC	0	2	3	5

3. Discussion

Since the first bibliometrics study on melatonin published two decades ago [110], melatonin has acquired more and more the role of a pleiotropic molecule, regulating each aspect of the biological clock, from sleep to appetite and reproduction. The great impact of this molecule on research is reflected by the highest number of citations corresponding to the 1990s and 2000s. Accordingly, the most frequent topics, found in the top-100 ranking, included sleep physiology, reproduction, circadian rhythms, and oxidative stress. These trends were also reflected by the content of the first ten most cited papers. Nonetheless, melatonin research, to date, covers a number of additional fields, besides the biomedical ones, which are expected to greatly contribute to the further importance of this molecule within the next years. Recently, melatonin has become a relevant issue in plant and food sciences [111,112], but we could not retrieve any specific article among the top 100 rank.

Like any other bibliometric study, our analysis is not exempt from a number of limitations. We are aware that other citation impact measures, not included in our analysis, also exist, such as the h-index, and also we did not control for the effects of self-citation. Additionally, in some cases, the number of citations cannot quantify the value of a work contribution to the field [113,114], since this is affected by many bias, mainly temporal ones [115]. Indeed, a paper tends to accumulate citations over time, while recent articles may not have had enough “publication time” to produce high rates in the citation analysis. Conversely, the number of citations may then fall progressively as the content of the paper is absorbed into the current knowledge. Moreover, our methodology was based on the Web of Knowledge platform, referring to all subscribed databases simultaneously consulted for the most comprehensive results. The Web of Science, however, does not index all peer-reviewed journals, thus we might have missed other journals indexed in other databases, such as Scopus. We did not use Google Scholar for this citation analysis, since despite being useful to cover some social and humanities sciences, is not accurate for the biomedical area. It has no quality control, searching within the web for scholarly content and considering, among the others, non peer-reviewed journals, books and academic theses, as well as non-scientific sites, such as promotional ones. The Google Scholar citation index is, thus, not considered highly reliable, at times pre-dating the publication it claims to cite and displaying manifold versions of the same publication, splitting the citation count [116].

4. Materials and Methods

In February 2016, we consulted Science Citation Index Expanded™, a specific online resource to quantify citations belonging to the Institute for Scientific Information (ISI) Web of Science™ platform [5]. Under “Basic Research” tag, the keyword (“topic”) used for search was “melatonin” and all the results were sorted using “time cited - highest to lowest”. A second search was then performed under the “Cited Reference Search” tag, using the word “melatonin” as “cited title”: every record were checked to identify the most cited ones and matched with the previously obtained list. The number of citations corresponded to the “Citing Article Counts”, which referred to all databases and all years, i.e., Web of Science™ Core Collection (1985-present), CABI, CAB Abstracts® (1973-present), Inspec® (1969-present), KCI-Korean Journal Database (1980-present), MEDLINE® (1950-present), SciELO Citation Index (1997-present). We deliberately excluded Google Scholar since it is not purposely intended to retrieve citations in a systematic and controlled way, as Web of Science or Scopus do [116]. The resulting 100 most cited articles were selected and full-text retrieved to verify the coherence with the topic (melatonin in research). The following data were recorded for each one: ranking based on the number of citations; number and names of the authors; year of publication; journal in which published and the corresponding 2013–2014 Journal Citation Report - Science Edition impact factor. The type of article was recorded (review, basic science or clinical trial) as well as methodological design (in vitro study, animal study, case-report, case series, narrative review/expert opinion, observational study, randomized clinical trial, systematic review/meta-analysis). No exclusion criteria were applied. Meanwhile, to further confirm the citation results, the Scopus™ database was also consulted. For each article, the annual citation rate (ACR) was calculated as the ratio between the number of citations (C) and the number of years (Y) since its publication: ACR = C/Y.

5. Conclusions

Within its limitations, this work highlight and confirms the increasing importance of melatonin, which, in perspective, is expected to significantly regulate the rhythm of future research, with predictable new trends going to be related to biomedical and nutritional sciences.