Hi Oak

Hope you find the root cause of your AS and beat it.
Interesting observation w.r.t depression. You might be interested in the following:

July 28, 2004: The Vitamin D Council: Dr. Reinhold Vieth and his colleagues at the University of Toronto have given hope to millions of people across the world who are suffering from the debilitating symptoms of depression.

In a paper published this week, Vieth and co-workers showed that vitamin D significantly improves mood and helps relieve the symptoms of depression in patients with vitamin D levels most doctors would mistakenly consider to be normal.
...

(My website: Lack of exposure to sunlight(UVB?) is the root cause of my ankylosing
spondylitis. Sunlight burnt my ankylosing spondylitis. Sunlight can be
deadly and can cause skin cancer, sun stroke etc.,. Safe limits are different
for different people. If you are vit D deficient, don't ignore it!)