However, people who wish to stay up later will want to use the lightbox early in the evening. Intensity can be changed simply by moving closer or further away from the light. Regular use and experimentation will determine the optimum time of day for each individual. Researchers at medical centers and clinics in the USA, Canada, and other countries around the world have had much success with light therapy in many hundreds of patients with clear histories of SAD for at least several years. Marked improvement is usually observed within four of five days, if not sooner.
Symptoms usually return in about the same amount of time when the lights are withdrawn.
Most users maintain a consistent daily schedule beginning, as needed, in fall or winter and usually continuing until spring, when outdoor light becomes sufficient to maintain good mood and high energy. Some people can skip treatment for between one to three days, but most notice the effects almost immediately if treatment is interrupted. Side effects have been minimal. People occasionally report eye irritation and redness which can be alleviated by sitting further away from the light. Using a humidifier to counteract the dryness of winter air indoors may also help.