The effects of ultraviolet-A wavelengths in light therapy for seasonal depression

Although light therapy is a recognized effective treatment for seasonal affective disorder (SAD), there has been little research into the critical wavelengths of light that produce the antidepressant effect. Previous studies found conflicting results for the importance of the ultraviolet (UV) spectrum in the therapeutic effect of light therapy. To assess the clinical effects of UV-A wavelengths (315-400 nm), we studied 33 depressed SAD patients diagnosed with structured interviews by DSM-IIIR criteria. Following a baseline week, patients underwent 2 weeks of 2500 lux light therapy for 2 h daily (06:00-08:00). Light therapy consisted of cool-white fluorescent light with the addition of a special UV-A fluorescent tube. Patients were randomized to wear glasses during light therapy that either blocked (UV-blocked condition) or passed (UV-A condition) wavelengths below 400 nm. Both treatments significantly reduced all depression ratings, but no differences were found between the UV-A and UV-blocked conditions. We conclude that the UV-A spectrum does not increase the antidepressant response of light therapy. Given the potential side effects of chronic UV exposure, clinical application of light therapy should use light sources that have the UV spectrum filtered.