Serotonin is a neurotransmitter that plays an important role in the regulation of the body by physiological and behavioral processes. It also has a role in controlling the action of the biological clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Serotonergic innervation comes from the central nucleus of the midbrain seamless and interacts with the other two main projections of up to SCN. The basic role of serotonin is to modulate the biological clock work by weakening moving light information reaching the SCN.