 Dear students, in this topic we shall discuss the pineal gland. This gland is a small reddish-gray gland which is found on the dorsal surface of forebrain of vertebrates. Its shape resembles that of a pine cone, so it is named as pineal gland. Dear students, the pineal gland is actually a modified photoreceptor. In some amphibians and reptiles, it is associated with their third eye or parietal eye, so it is linked to the light-sensing organs and has photoreceptors in it. In birds and mammals, it contains light-sensitive cells and has connections with the eyes. So, because of presence of light-sensitive cells in it, light controls its secretory activity. Dear students, the pineal gland produces a hormone which is called melatonin. This melatonin is synthesized from serotonin. Serotonin itself is synthesized from the amino acid tryptophan. This amino acid tryptophan is made from serotonin or serotonin. The hormone melatonin is involved in regulating the biorhythms in vertebrates. This hormone is produced due to light sense of light, light effects on the pineal gland as a result melatonin is produced. So the production of melatonin is stimulated by darkness while it is inhibited by light. So it is secreted at night and its amount released depends on the length of the night. During winter when days are short and nights are long, more melatonin is secreted. Melatonin is involved in the regulation of many rhythmic activities related to light and dark periods and the seasons involving the length of day and night periods. Dear students, the main target of melatonin is the suprachiasmatic nucleus in the hypothalamus. This suprachiasmatic nucleus of hypothalamus, it actually functions as a biological clock. The timing of all this is the result of this hypothalamus. Melatonin actually decreases the activity of this part or as a result this is related, this basic fact is related to the effect of melatonin in mediating biorhythms. Dear students, now we shall discuss the role of melatonin in sleep wake up cycles. Birds or mammals have very characteristic sleep or wake cycles. In these cycles melatonin plays an important role. It modulates the brain stem structures which govern the sleep wake up cycles. Melatonin's synthesis increases when light decreases and in the previous night it is at its peak. In childhood it is more produced. In elder age its production decreases. This is the reason that in elderly people who get disturbances in sleep cycles and their sleep decreases, that is why less production of melatonin occurs. Dear students, melatonin also regulates the seasonal fertility in many animals. Melatonin also causes a decrease in the production of gonadotropin-releasing hormone from the hypothalamus and gonadotropic hormones from the pituitary. As a result, gonad-activity of the gonads is inhibited. In many animals, gonads become involuted or shrink during certain seasons of the year. It commonly happens during the early winters when nights are longer. When winter ends and spring season is about to come, at that time the secretion of gonadotropin resumes and increases. As a result, animals and their gonads become prepared for springtime reproductive activity. Dear students, melatonin is also related to the sexual development in humans. In children, melatonin is produced in much larger quantities. This is considered as the reason that gonads do not develop in children and sexual development does not happen. When puberty comes, melatonin production reduces. In adults, melatonin is also involved in controlling sex drive and reproduction in humans.