melatonin: hormone secreted by the pineal gland
hormone: the secretion of an endocrine gland that is transmitted by the blood to the tissue on which it has a specific effect
low-density lipoprotein: a lipoprotein that transports cholesterol in the blood; composed of moderate amount of protein and a large amount of cholesterol; high levels are thought to be associated with increased risk of coronary heart disease and atherosclerosis