Melatonin, often known as the sleep hormone, is a hormone released by the pineal gland in the brain and helps maintain your sleep-wake cycle. Your body's internal clock and the level of light you are exposed to can influence when and how much melatonin is released into your body. Generally, melatonin levels in the body starts to rise after the sun has set, in the mid-to-late evening and is a signal to your body that it is time to go to sleep. Melatonin is also found naturally in some foods, such as walnuts and may also be available in nutritional supplements.