Researchers at more than 15 medical centers and clinics in both the U.S. and abroad have had much success with light therapy in patients with clear histories of SAD for at least several years. Marked improvement is usually observed within a week, if not sooner, and symptoms usually return in about the same amount of time when the lights are withdrawn. Most users, therefore, maintain a consistent daily schedule beginning, as needed, in fall or winter and usually continuing until the end of April, by which time outdoor light is sufficient to maintain good mood and high energy. Some people can skip treatments for one to three days, occasionally longer, without ill effect, but most start to slump quickly when treatment is interrupted.