Data from a large number of controlled clinical trials conducted during the last twenty years has repeatedly demonstrated the clinical efficacy of light to treat SAD. Response rates have equaled or exceeded commonly used antidepressant drugs. In 1998, the Archives of General Psychiatry published the results of two-large scale controlled trials conducted by over three-to-four year periods demonstrating the positive results of light therapy. The studies are even more compelling than earlier trials because of the large numbers of patients involved and the use of ion generators as a placebo control. The difficulty of finding a placebo for bright light led earlier researchers to use lower intensity light as a placebo, and patient response to this lower intensity light was relatively frequent. While it is quite likely that the results occurred because lower level light is therapeutic for some patients, later researchers searched for an effective placebo. Clinical trials have convinced even the most skeptical scientists that light therapy can be effective.