Exposure to light at night (LAN), such as sleeping with a nightlight, suppresses the production of melatonin, which is produced from the amino acid tryptophan by the pineal gland when the eyes detect no light (i.e., in darkness or blindness, or during sleep). [Through a series of enzyme-catalyzed reactions, tryptophan is partially converted to 5-hydroxytryptophan (5-HTP), which is partially converted to serotonin, which is partially converted to melatonin.] Melatonin also is produced by the retina and, in vastly greater amounts, by the gastrointestinal system. Increased LAN has been connected with leukemia and breast cancer, likely because it disrupts circadian rhythms. Latin for "about one day," circadian rhythms help coordinate and synchronize our internal body functions. Deep within the brain, in the anterior hypothalamus, lies the "suprachiasmatic nucleus" (SCN), a dual cluster of thousands of nerve cells. This is the body's "pacemaker" or "biological clock," which is powered, in a sense, by light. The SCN receives signals from the outside world via the retina of the eye. When the SCN is stimulated by daylight signals from the retina, it tells the pineal gland to suppress production of melatonin. At night, the SCN's lack of stimulation is signaled to the pineal gland, and melatonin secretion is increased many times over, creating a physiological condition of "biological night" in the person. Why is melatonin so important? Melatonin has been shown to by cytotoxic and induces apoptosis (cancer cell auto-destruction). In addition to destroying cancer cells and inhibiting proliferation, it also stops angiogenesis (new tumor blood vessel growth) and prevents harmful forms of estrogen from stimulating cancer cell growth. Melatonin also stimulates the immune system and increases the cancer-killing activity of macrophages, monocytes, natural killer cells, T-helper cells and eosinophils, all of which are involved in cancer cell destruction.