Melatonin is a hormone produced by the pineal gland in the hypothalamus of the brain. It is involved in regulating the reproductive and sleep cycles. Melatonin plays a key role in regulating the internal body clock, telling the body what time of day it is and when we need to eat, sleep, etc.
Melatonin is released in response to darkness, and acts on the brain to promote sleep. Daylight inhibits melatonin secretion, thus encouraging daytime activity. The amount of melatonin in the body is disturbed during long-haul air travel and is responsible for causing the symptoms of jet lag.