There is much more seasonal difference in higher latitudes than in lower latitudes. In a significant portion of the population of the northern United States, the shorter days of fall and winter precipitate a syndrome that can consist of depression, fatigue, hypersomnolence, hyperphagia, carbohydrate craving, weight gain, and loss of libido. If these symptoms persist in the winter, abate as the days grow longer, and disappear in the summer, the diagnosis of seasonal affective disorder (SAD) can be made. Many hypotheses exist regarding the biochemical mechanisms behind the predisposition toward this disease, including circadian phase shifting, abnormal pineal melatonin secretion, and abnormal serotonin synthesis. Although the mechanism(s) behind this disease is not fully known, one treatment appears to address each of the theories. Light therapy is a natural, non-invasive, effective, well-researched method of treatment for SAD. Various light temperatures and times of administration of light therapy have been studied, and a combination of morning and evening exposure appears to offer the best efficacy. Other natural methods of treatment have been studied, including L-tryptophan, Hypericum perforatum (St. John's wort), and melatonin.