Here is a comprehensive guide to light therapy by a renowned and practicing psychiatrist. In short:
Light therapy can treat seasonal affective disorder (eg winter depression) and sometimes regular depression. Buy this light box [see link below], and at some time between 6 AM and 9 AM, sit exactly 12 inches away from it and do some activity that doesn’t involve staring directly at the light box. Continue every morning for the period of time you’re at risk of depression. If bipolar, don’t try this without medical supervision.
It is detailed, easy to read and understand (no medical jargon), and no-nonsense.
If you’ve read this document, you already know more than 95% of psychiatrists about light therapy. Trust me, I’ve given lectures about this to psychiatrists and they don’t know any of this stuff.
The linked light box: Carex Day-Light Classic Plus Bright Light Therapy Lamp