I haven't read the whole chapter yet, but so far he's talking about some light receptors in the eye that need to be stimulated. It seems like it's important to have the light on the eyes. Not shining directly in, of course, but at the right angle to be sure to reach those receptors.

I haven't seen a mention of vitamin D yet. But my son read this book about 6 months ago, and he went out and bought some vitamin D supplements at that time, so maybe it's covered somewhere in the book.

I agree with jimi. For vitamin D production you need one of the components of natural sunlight. UV B? I can't remember. I just remember that even a glass window will filter most of it out. They do make special light bulbs that emit the right rays for vitamin D. Pet reptiles need them. And some people with some skin disease use them. And some tanning salons. But that's different from the light used for depression which seems to have more to do with brightness.