Background: Melatonin, a neurohormone synthesized primarily in the pineal gland, plays a central role in regulating circadian rhythms and sleep-wake cycles. Purpose: Beyond its chronobiotic functions,melatonin exhibits potent a ntioxidant,anti inflammatory, immunomodulatory,and oncostatic proper ties.This review explores the multifaceted molecular and cellular mecha nisms of melatonin across physiological and pathological conditions. Methods: A systematic review following PRISMA guidelines was conducted using databases including PubMed, Scopus, Web of Science, and Google Scholar. Studies from 1990 to 2024 focusing on mechanisti c insights were included.