The pineal gland is a reddish-brown bean-shaped body located between the precuneus and thalamus of the midbrain. Pineal cells can synthesize and secrete a variety of substances, mainly the following functions: 1. Secretion of melatonin: The pineal gland can secrete melatonin, and the secretion concentration is regulated by circadian changes, low during the day and high at night, which can be used as a signal from the central nervous system to control many biological clock phenomena. Melatonin also influences sleep, mood, and more. Melatonin can also inhibit the synthesis and secretion of gonadotropins and their related hormones, and regulate the function of the reproductive system. 2. Synthesis of hormones: The pineal gland can synthesize peptide hormones such as gonadotropins, thyroid stimulating hormones, 8-arginine oxytocin, etc., which regulate many physiological functions of the human body.