Melatonin is a neuroendocrine hormone involved in circadian rhythm regulation, sleep-wake timing, antioxidant signaling, seasonal physiology, immune communication, and mitochondrial protection. The hormone functions as an internal timing signal that helps synchronize physiological processes with environmental light-dark cycles. Melatonin influences sleep initiation, body temperature rhythms, endocrine timing, metabolic coordination, and circadian gene expression throughout the body.
In addition to its role in circadian regulation, melatonin participates in antioxidant defense systems and mitochondrial homeostasis. It can influence oxidative stress pathways, inflammatory signaling, and cellular repair processes. Melatonin receptors are present in the brain, retina, cardiovascular tissues, immune cells, liver, gastrointestinal tract, and reproductive organs, allowing widespread physiological coordination.
Melatonin is produced mainly by the pineal gland from the amino acid tryptophan through serotonin intermediates. Tryptophan is converted into serotonin, which is then acetylated and methylated by arylalkylamine N-acetyltransferase and hydroxyindole O-methyltransferase to form melatonin.
Production rises during darkness and declines in response to light exposure. Specialized retinal ganglion cells detect environmental light and transmit signals to the suprachiasmatic nucleus, which controls sympathetic innervation of the pineal gland. Smaller amounts of melatonin may also be synthesized within gastrointestinal tissue, immune cells, retina, and mitochondria.
Melatonin secretion is regulated primarily by circadian signaling pathways and environmental light exposure. Darkness stimulates sympathetic signaling to pinealocytes, increasing melatonin synthesis, while light suppresses production. Circadian timing signals from the suprachiasmatic nucleus coordinate nightly release patterns.
Melatonin acts through MT1 and MT2 receptors, influencing cyclic AMP pathways, circadian clock gene regulation, neuronal excitability, and endocrine timing systems. Sleep patterns, aging, artificial light exposure, shift-work schedules, stress physiology, and inflammatory signaling can alter secretion rhythms. Through these integrated chronobiological and endocrine pathways, melatonin synchronizes physiological timing, supports circadian stability, and contributes to oxidative and mitochondrial regulation during sleep-associated recovery periods.
Indoleamine made from serotonin at night that signals circadian “darkness” via MT1/MT2 receptors.
