Melatonin is just a very small part of all the variables involved in sleep. If you are excited or stressed you'll have high levels of adrenaline, cortisone, or cortisol. Diet will affect levels of ghrelin, leptin and insulin. Low testosterone levels, or antidiuretic hormones, can be found in many people. All these things will affect your sleep just as much, if not more, then melatonin does.