Light therapy is often incorrectly likened to the secretion of vitamin D.
Indeed, vitamin D is synthesized by cells in the skin when these are in contact with UVB and UVA which are naturally found in sunlight. A light therapy lamp, if it is certified, emits a light whose spectrum is completely devoid of ultra-violet.
That is why the skin does not emit vitamin D during a light therapy session.
On the other hand, the exposure of your eyes to a light therapy lamp has beneficial effects on the regulation of your natural rhythms and your energy level.