How To Tackle Winter Blues With Bright Light Therapy Bright Light Therapy (BLT) corrects circadian rhythm phase delay. BLT treatment in the morning shifts the circadian rhythm earlier, which leads to less discordance between the circadian rhythm and the actual clock time. This helps treating seasonal affective disorder (SAD). Introduction to Bright Light Therapy In the pioneering study on SAD (see below) in 1984, Rosenthal and his colleagues used bright white light to mitigate seasonal affective disorder symptoms. They used white full-spectrum fluorescent light (2,500 lux) for 3 hours at dawn (the time of morning when the Sun is 6° below the horizon) and hours at dusk for two...