melatonin

a hormone produced by the pineal gland in darkness but not in bright light. Melatonin receptors in the brain, in a nucleus immediately above the optic chiasma , react to this hormone and synchronize the nucleus to the 24-hour day/night rhythm, thus informing the brain when it is day and when it is night. Melatonin is a derivative of serotonin , with which it works to regulate the sleep cycle, and is being used experimentally to treat jet lag, SADS , and insomnia in shift workers and the elderly.