Melatonin Receptor Agonists in Sleep Disorders: An Evidence-Based Review of Circadian Biology, MT1 and MT2 Receptor Pharmacology, Clinical Effectiveness, and Future Therapeutic Directions

Sleep disorders are prevalent conditions associated with impaired daytime functioning, cognitive disturbances, mood alterations, and reduced quality of life. The regulation of sleep is closely associated with the circadian timing system, in which melatonin acts as an important endogenous signal of darkness. The physiological effects of melatonin are primarily mediated through two G-protein-coupled receptors, MT1 and MT2. Pharmacological agents that activate these receptors have therefore emerged as therapeutic options for insomnia and circadian rhythm sleep–wake disorders. This review discusses the physiological role of melatonin, the molecular mechanisms associated with MT1 and MT2 receptor activation, and the pharmacological characteristics of major melatonergic agents, including melatonin, ramelteon, tasimelteon, and agomelatine. Their clinical applications in insomnia disorder, delayed sleep–wake phase disorder, Non-24-Hour Sleep–Wake Disorder, jet lag, and sleep disturbances associated with psychiatric conditions are reviewed. Although these agents share a common melatonergic mechanism, they differ in receptor affinity, pharmacokinetic properties, clinical indications, and therapeutic effects. Melatonin is particularly important as a chronobiotic agent, whereas ramelteon is primarily associated with sleep-onset insomnia. Tasimelteon has a specific role in circadian entrainment, particularly in Non-24-Hour Sleep–Wake Disorder. Agomelatine is pharmacologically distinctive because it combines MT1/MT2 receptor agonism with 5-HT2C receptor antagonism and may be useful in patients with depressive illness accompanied by sleep disturbance. Overall, melatonin receptor agonists provide a pharmacologically targeted approach to sleep and circadian disorders with generally favorable tolerability and low abuse potential compared with conventional hypnotics. However, their efficacy is influenced by the underlying sleep disorder, dose, formulation, administration timing, and patient characteristics. Future research should focus on personalized chronotherapy, circadian biomarkers, receptor-selective ligands, and individualized treatment strategies.