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Seasonal affective disorder
Full sunlight or exposure to bright light from a light box is used to treat seasonal affective disorder (SAD). Light boxes for SAD are designed to filter out most UV light, which can cause eye and skin damage. Mayo Clinic states that light therapy is of proven effectiveness for treating seasonal affective disorder and light therapy is seen as its main form of treatment. Controlled-trial comparisons with antidepressants show equal effectiveness, with less expense and more rapid onset of therapeutic benefit, though a minority of patients may not respond to it. Direct sunlight, reflected into the windows of a home or office by a computer-controlled mirror device called a heliostat, has also been used as a type of light therapy for the treatment of SAD.

The effectiveness of light therapy for treating SAD may be linked to the fact that light therapy makes up for lost sunlight exposure and resets the body's internal clock. Studies show that light therapy helps reduce the debilitating and depressive behaviors of SAD, such as excessive sleepiness and fatigue, which results lasting for at least 1 month. Light therapy is preferred over antidepressants in the treatment of SAD because it is a relatively safe and easy therapy.

It is possible that response to light therapy for SAD could be season dependent. Morning therapy has provided the best results because light in the early morning aids in regulating the circadian rhythm.