In the human body, melatonin is released by the pineal gland and helps regulate sleep timing.
Melatonin production usually rises in darkness and falls in bright light. The pineal gland, a small endocrine structure near the centre of the brain, releases it into the bloodstream. Melatonin is part of the body's circadian timing system, which coordinates daily patterns such as sleepiness and wakefulness.
Melatonin does not function as a simple knockout switch, and its effects vary among people. Light reaching the eyes influences the suprachiasmatic nucleus, the brain's main circadian clock, which then helps control pineal melatonin production. Artificial light at night can alter this signal. Melatonin is different from adrenaline, which supports rapid stress responses, and insulin, which helps regulate blood glucose. The hormone is also found in many animals and plants.