Melatonin is an endogenous hormone synthesized primarily by the pineal gland and plays a central role in regulating the body’s circadian rhythm by signaling the transition from wakefulness to sleep in response to diminishing light. Its secretion follows a tightly controlled, light-dependent pattern governed by the suprachiasmatic nucleus (SCN) of the hypothalamus, where retinal input suppresses melatonin during daylight and permits its evening rise, inducing a state of physiological relaxation and reduced arousal that facilitates sleep onset. Beyond its circadian timing function, melatonin also contributes to broader neuroendocrine and metabolic regulation, influences core body temperature reduction prior to sleep, and exhibits antioxidant and anti-inflammatory properties in various research models. Over-the-counter melatonin supplements leverage these biological actions by providing exogenous hormone support, which can help accelerate sleep initiation, extend total sleep time, and improve sleep quality, particularly in contexts where natural circadian signaling is disrupted, such as insomnia, jet lag, shift work, or delayed sleep-wake phase disorders. Research continues to investigate melatonin’s pharmacokinetics, optimal dosing strategies, and potential therapeutic roles beyond sleep regulation, including its influence on mitochondrial function, immune modulation, and neuroprotective pathways.