Do I need to undress during light therapy to have an effect?

Yes, it can be an advantage to undress during light therapy. In particular, the red light is easily stopped by clothing, while the near infrared can actually penetrate lighter materials. You can easily see if the garment you are using lets through any red light as it will appear as a red glow. Regardless, the clothing will slightly reduce the effect of the light therapy, but if you compensate with a longer treatment time, it may still be sufficient. There may be times when it is actually justified to wear lighter clothing, so better a treatment with light clothing than none at all.