Exposure to light results in a rapid breakdown of melatonin. Circadian rhythms can also be monthly, as in menstrual cycles or seasonal, as in mammalian mating patterns. Melatonin is responsible for the entrainment of many of your body's hormonal secretions. There is also an output pathway, completing a feedback cycle, that informs your circadian periods, such as when you choose to go to bed and how much time you spend outside. The nocturnal period of melatonin secretion is inversely proportional to daylength.
Stress disrupts the circadian rhythms of the body, as does depression.
When melatonin enjoyed its fifteen minutes of fame a few years ago, popular press touted its anti-aging properties. Their argument was fallacious on two accounts, one logical, another factual. Factually, a gradual reduction of melatonin with aging is NOT a universally accepted aspect of aging. At least one study we've read revealed that healthy elderly subjects have the same melatonin levels as young adult subjects 13 . That factual presupposition led to the logically incorrect argument that if melatonin decreases with age, supplementing melatonin can halt or slow the aging process.