Answer
Melatonin--derived from serotonin-- is a hormone secreted by the pineal gland. It contributes to the setting of the "body clock' which is controlled by the suprachiasmatic nucleus of the hypothalamus.
Work Step by Step
Because more melatonin (X 10) is secreted by the pineal in darkness than in light, it is believed that one of the functions of this hormone is to induce sleep. In sleep medicine, small amounts of melatonin are sometimes prescribed to help reset a patient's sleep (circadian) rhythm.
