Someone wrote and questioned my position on SAD. As I told them, note that I said "likely" not absolutely. I am sure many if not most doctors would agree with that assessment. The truth is that SAD is a fairly rare physical issue, from all that I can tell, including the resources that you cite. It does stem from physical issues, but not nearly so often as people might think.
Remember that most doctors are not equipped to deal with non-physical problems. They have a vested interest in keeping you on the hook of physical problems/solutions to perpetuate their own ends. Furthermore, most doctors are very unfamiliar with the immaterial/spiritual part of man and therefore they can't address it. They cannot address with medicine the lack of belief, the failure to bring every thought captive to Christ, etc. They are simply ill-equipped. When these are the actual problems, they can't fix them. So while these things are not always the cause, when they are, we have to address them.
So while depression and SAD can be physical in nature, they are often not physical in nature, and that's all I have said.
Remember, my caution is against the simplistic model that so many subscribe to that says, "He feels bad, medicate him." In fact, it may be an entirely different solution that is necessary.
I have never said that there are no physical causes, such as vitamin deficiency, etc. What I have said is that the complexity of the human existence means we don't always know precisely the cause and effect. It may be the certain patterns of thought trigger physical reactions rather than physical causes triggering patterns of thought. That is the factor that is quite frequently overlooked. In times of stress, our bodies process differently. So if I have a case of mental depression, I should expect my body to need more sleep, to feel less like doing things, etc. I can overcome that with vitamins, meds, even caffeine, but I haven't addressed the real issue which is the thought process.
So I am urging us to be more complete in our thinking on this issue, not less. We have to take into full account the complexities of human existence. We cannot abide by the simplicities. So many have such a narrow view of this, and that is unfortunate.