Is it Depression or a Dark Night of the Soul? is a helpful article that I found via Carmelite Conversations.
Even though St. John of the Cross is the go-to for reading about Dark Night, I thought of St. Teresa of Avila when I finished reading this. She wrote in various places about people suffering nervous disorders, so she would likely approve of a contemporary look at the differences between depression and spiritual aridity.
An article that links off of this article is Am I depressed or just deep? Well worth a read, as well. The author had me chuckling at the recollections of my teenaged self. The Then-Me thought I was being philosophical and deep; I was really suffering a self-imposed depression. I call it self-imposed depression because my mood back then wasn't anything that could have been helped by the medical profession. Rather, I needed Me from the future to travel back in time and tell Then-Me to grow up a bit, stop being so self-centered, eat properly and get enough rest, and things would be a lot better. Ah, well. At least I am finally able to chuckle at Then-Me. She has been haunting me for years!