Melatonin is a hormone produced by the brain that regulates normal sleep-wake cycle. Often, it is termed as the "sleep hormone".
The body's production of melatonin depends on the exposure to light and darkness. Melatonin is released when the environment is dark (i.e. night) to help one feel relaxed and fall asleep. At daybreak, the level of melatonin drops. In other words, bright light inhibits the production of melatonin therefore it is advised to reduce the use of devices before bed every night. In addition, melatonin declines with age.
Melatonin is typically available in the form of supplement to help relieve sleep problems, such as insomnia, jetlag, shift work disorder and for eldery, who has difficulties falling sleep due to decreased melatonin level.