Somewhat related to #1 is vitamin D deficiency. Don't fall in the trap of dismissing it because it's "just a vitamin" (which it actually isn't), because a severe deficiency will screw you up good. Pretty much everyone is deficient, but the severity varies. Geeks who spend most of their time indoors, staying late up at night working on stuff are more at risk. Don't assume that drinking a lot of fortified milk/whatever or living in a sunny climate makes you immune, because it doesn't.

At first a deficiency will just make you feel "off", but if left untreated it will make you feel as if you're slowly dying (not exaggerating) with muscle/bone/joint pain, fever, blurry vision, insomnia and brain fog. Depression and anxiety are secondary symptoms (how can you not become depressed with all this?), but still just symptoms, not a cause (in this case).

It's a simple test that could save you (anyone) a lot of grief, because you really do not want to let it go that far, and recovery takes a looooong time (months). You may have to insist (and do insist) on getting it checked, because most doctors won't by default. Most doctors also don't know how to interpret test results properly. <30 ng/mL is deficient, and ideally it should be 60-70.

Also worth considering is taking a B-complex vitamin. But first and foremost, talk to a doctor about it.