Our circadian cycles -- that is, our internal "body clocks" -- determine our daily sleep cycles, performance, alertness, moods, and even our gastrointestinal functions and metabolism. Melatonin, a hormone made by the pineal gland in the base of the brain, is linked to the circadian system. Along with sunlight, melatonin helps to set the brain's biological clock. At night, melatonin is secreted, causing the body temperature to lower, and helping us sleep.