Melatonin is a methoxyindole synthesized and secreted principally by the pineal gland at night under normal environmental conditions. The endogenous rhythm of secretion is generated by the suprachiasmatic nuclei and entrained to the light/dark cycle. In the absence of light, a tiny cone-shaped structure in the brain, known as the pineal gland, generates and secretes melatonin; this secretion results in the onset of physiological changes associated with sleep. In vertebrates, melatonin is produced in darkness, thus usually at night, by the pineal gland, a small endocrine gland [96] located in the center of the brain but outside the blood–brain barrier. The main function of the pineal gland is to receive and convey information about the current light-dark cycle from the environment via the production and secretion of melatonin cyclically at night (dark period) (6, 7). One function of the pineal gland is to produce melatonin. Melatonin has various functions in the central nervous system, the most important of which is to help modulate sleep patterns. The pineal gland is a small, pea-shaped endocrine gland located in the brain, responsible for producing the hormone melatonin, which regulates sleep-wake cycles and circadian rhythms. The main function of the pineal gland is to receive information about the state of the light-dark cycle from the environment and convey this information by the production and secretion of the hormone melatonin. Pineal gland, endocrine gland found in vertebrates that is the source of melatonin, a hormone derived from tryptophan that plays a central role in the regulation of circadian rhythm (the roughly 24-hour cycle of biological activities associated with natural periods of light and darkness). The pineal gland plays a central role in controlling circadian rhythms through the nighttime secretion of melatonin. 2 Despite this gland’s importance, progress in understanding human pineal development has been limited by its deep-brain location, ethical barriers to obtaining human tissue, and species differences in melatonin biology. Organoids derived from human pluripotent stem cells. Melatonin’s presence is a fundamental component of the body’s daily physiological changes. The pineal gland, a small endocrine gland shaped like a pinecone, is the primary site for melatonin secretion in the brain.