An actively functioning endocrine gland, located in the brain, which secretes melatonin, is strongly regulated by light stimuli, and is an important component of the circadian timing system. The pineal gland or pineal body has the Latin name epiphysis cerebri. It is virtually ubiquitous throughout the vertebrate animal kingdom. In nonmammalian vertebrates (lampreys, fishes, amphibians, reptiles, birds), it functions as a photoreceptive third eye and an endocrine organ. In mammals, it serves as an endocrine organ that is regulated by light entering the body via the paired eyes. In all vertebrates, including humans, the pineal gland produces the hormone melatonin, with the highest synthesis and secretion occurring during the night. Despite extensive species variation in anatomy and physiology, the pineal gland generally serves as an essential component of the circadian system which allows animals to internally measure time and coordinate physiological time-keeping with the external environment.