Melatonin (N-acetyl-5-methoxytryptamine) is a neurohormone synthesized by the pineal gland during the dark period in all animal species and immediately released. The rhythm of melatonin synthesis is directly controlled by photoperiod. Melatonin secretion both in blood and in cerebrospinal fluid transduces the photoperiodic message to all central or peripheral structures expressing melatoninergic receptors or binding sites, allowing synchronization of several cellular and physiological events to a given day-length. Melatonin rhythm synchronizes a wide range of physiological functions including among others circadian rhythms, seasonal reproduction, immunity etc. Anarchic melatonin secretion also accounts for disorders of sleep/wake rhythms associated or not with depression. Fluorescent melatoninergic ligands (Viault, G., et al. RSC Adv. 2016, 6, 62508–62521) were designed by associating the 4-azamelatonin ligands with different fluorophores. The ligands show good affinities for MT1 and/or MT2 receptors and substitution of the fluorophore at positions 2 or 5 of the azamelatonin core had a direct impact on the MT receptors selectivity while grafting the fluorophores on position N-1 produced fluorescent ligands with good affinities for both MT1/MT2 receptors. The optimal position N-1, C-2 or C-5 on the 4-azamelatonin ligand appeared strongly dependent upon the nature of the fluorophore itself.