Pineal cells were dominated by sympathetic postganglionic fibers which from cervical ganglion,sympathetic stimulation may promote the synthesis and secretion of pineal melatonin.Secretion of the pineal gland is closely related to light, pineal gland will become small by continuously lighting, inhibit the secretion of pineal cells, and dark on the secretion of the pineal gland play a catalytic role.Since melatonin secretion and synthesis are regulated by light and darkness, so it appears secretion circadian rhythm. In human plasma, its secretion is lowest at noon, and highest at midnight.In addition, its cyclical secretion is closely related to the sexual cycle of animals and humans, as well as to the menstrual cycle of women.Pineal gland will release [time signal] to the central nervous system through melatonin secretion cycle, thus affecting the body's biological effects of time, such as sleep and awakening, especially the cyclical activity of hypothalamus-pituitary-gonadal axis.