Bright white fluorescent light has been shown to reverse the winter depressive symptoms of SAD. Early studies used expensive "full-spectrum" bulbs, but these are not especially advantageous. Bulbs with color temperatures between 3000 and 6500 degrees Kelvin all have been shown to be effective. The lower color temperatures produce "softer" white light with less visual glare, while the higher color temperatures produce a "colder" skylight hue. The lamps are encased in a box with a diffusing lens, which also filters out ultraviolet radiation. The box sits on a tabletop, preferably on a stand that raises it to eye level and above. Such an arrangement further reduces glare sensations at high intensity, and preferentially illuminates the lower half of the retina, which is rich in photoreceptors that are thought to mediate the antidepressant response. Studies show between 50% and 80% of users showing essentially complete remission of symptoms, although the treatment needs to continue throughout the difficult season in order to maintain this benefit.
There are three major dosing dimensions of light therapy, and optimum effect requires that the dose be individualized, just as for medications.
• Light intensity. The treatment uses an artificial equivalent of early morning full daylight (2500 to 10,000 lux), higher than projected by normal home light fixtures (50 to 300 lux). A light box should be capable of delivering 10,000 lux at eye level, which allows downward adjustments if necessary.
• Light duration. Daily sessions of 20 to 60 minutes may be needed. Since light intensity and duration interact, longer sessions will be needed at lower intensities. At 10,000 lux – the current standard – 30-minute sessions are most typical.
• Time of Day of exposure. The antidepressant effect, many investigators think, is mediated by light’s action on the internal circadian rhythm clock. Most patients with winter depression benefit by resetting this clock earlier, which is achieved specifically with morning light exposure. Since different people have different clock phases (early types, neutral types, late types), the optimum time of light exposure can differ greatly. The Center for Environmental Therapeutics, a professional nonprofit agency, offers an on-line questionnaire on its website, www.cet.org, which can be used to calculate a recommended treatment time individually, which is then adjusted depending on response. Long sleepers may need to wake up earlier for best effect, while short sleepers can maintain their habitual sleep-wake schedule.
Side effects of light therapy are uncommon. Some patients complain of irritability, eyestrain, headaches, or nausea. Those who have histories of hypomania in spring or summer are at risk for switching states under light therapy, in which case light dose needs to be reduced. There is no evidence for long-term adverse effects, however, and disturbances experienced during the first few exposures often disappear spontaneously. As an important precaution, patients with Bipolar I disorder – who are at risk for switching into full-blown manic episodes – need to be on a mood-stabilizing drug while using light therapy.