Melatonin
Melatonin is a natural hormone made by your body's pineal (pih-knee-uhl) gland. This is a pea-sized gland located just above the middle of the brain. During the day the pineal is inactive. When the sun goes down and darkness occurs, the pineal is "turned on" by the SCN and begins to actively produce melatonin, which is released into the blood. Usually, this occurs around 9 pm. As a result, melatonin levels in the blood rise sharply and you begin to feel less alert. Sleep becomes more inviting. Melatonin levels in the blood stay elevated for about 12 hours - all through the night - before the light of a new day when they fall back to low daytime levels by about 9 am. Daytime levels of melatonin are barely detectable.
http://sleepfoundation.org/sleep-topics/melatonin-and-sleep
What is melatonin?
Melatonin is a hormone produced in the brain that helps regulate your sleep and wake cycle.
Melatonin is most commonly used to treat insomnia, jet lag, and to aid in giving up smoking or long-term use of anti-anxiety medicine (Valium). It is also used to help regulate sleep/wake cycles in people who are blind.
Melatonin has also been used in alternative medicine as an aid to treat insomnia caused by Alzheimer's disease or depression.
http://www.drugs.com/melatonin.html
In humans, its
primary role is that of a circadian pacemaker,
and exogenous melatonin has a welldocumented
soporific effect when taken during
the daytime or early evening
successfully used to treat various sleep-cycle
disturbances in children with chronic sleep
disorders by decreasing sleep onset latency
Impulse: The Premier Journal for Undergraduate Publications in the Neurosciences
2009
People use melatonin to adjust the body's internal clock. It is used for jet lag, for adjusting sleep-wake cycles in people whose daily work schedule changes (shift-work disorder), and for helping blind people...