What Is The Pineal Gland? The pineal gland is one of the most mysterious glands of the human body. It is a pea-sized gland in the precise geometric center of the brain and comes from the root word “pinea” which is Latin for “pinecone”. It is responsible for the synthesis and secretion of the hormone melatonin. Melatonin maintains the body’s circadian rhythm (sleep-wake cycle), regulates the onset of puberty in females, helps protect the body from cell damage caused by free radicals and much, much more.