One theory is that serotonin levels drop off in winter. Another is that melatonin secretion may be the cause. Yet another theory posits that circadian rhythms are thrown out of whack. One thing that science agrees on is that light therapy is the treatment of choice, and that the eye and not the skin responds to the light. A light box is the standard device, one that produces at least 2,500 lux at a table or eye level, but light visors are also effective.
Patients keep their eyes open and glance toward the light, but avoid staring directly into it.
Sessions should start at 10 or 15 minutes a day, and gradually increase to 30 to 45 minutes a day in the morning or evening. Maximum duration is 90 minutes, though many trials have used durations of two hours or more. The most common side effects are eyestrain and headache. The major disadvantage is the time commitment, that is unless you can set up your light box at your work station or while you're eating a meal or reading the paper. A number of studies have found morning treatment to be more effective than evening treatment, possibly due to coinciding with the dawn.