Melatonin is a hormone that regulates your sleep-wake cycle. Melatonin is secreted by the pineal gland (located in the brain) and helps to control circadian rhythms, or your internal clock. The production of melatonin is triggered by darkness and inhibited by light. So if you are exposed to bright light in the evening (like from a computer screen or TV), melatonin production will be suppressed, making it harder for you to fall asleep at night.