The pineal gland is a small endocrine gland named for its pinecone shape. This gland is located on the roof of the third ventricle within the forebrain’s diencephalon region. It is made of secretory cells called pinealocytes arranged in compact cords and clusters surrounding dense particles of calcium salts. The pineal gland secretes the hormone melatonin, an amine derivative of serotonin. The melatonin concentration in the blood is linked to light exposure and follows the day-night pattern. The secretion peaks at night when the photoreceptors in the eye do not detect light, leading to drowsiness. In addition to regulating sleep, melatonin has antioxidant effects and mediates mating behavior by altering the timing and duration of the reproductive cycle, puberty, and sexual maturation.