One interesting correlation is high intelligence. It seems to be a price that some among the super-intelligent must pay. I don't know how the genetic link occurs, or if anyone knows, but this observation has been made.
My wife and I are very functional, but when sleep deprived, the tank (good analogy, Mars), runs out. After a good night's sleep, then everything is back in order. Our tendencies for this seem to have compounded in our children, and all three of them carry an increased burden.
Our story goes very much the same as uncoug's post above. It was bewildering and frustrating to see this happen, and it took us a long time and a lot of counseling to finally get it sorted out. One of our kids was hit hard by this disorder when he was 14 years old. He tried to compensate by acting cool and non- chalant, but before long, he could not think his way through his school work, and eventually could not get out of bed. He had been a straight A student before, and he did not know what was happening to him. He kept trying to blame his struggles on external events or circumstances. He was in denial because he was terrified of the possibility that the problem had arisen within him.
Eventually, we found a particularly insightful counselor who told him that the depression was not his fault, and it did not define him. He referred our son to a pediatric psychiatrist. Several therapeutic trials were unsuccessful, but finally some partial relief was afforded by pharmacological manipulation. It was as though our son had been semi comatose and then finally woke up. His old self re- emerged.
It is still very difficult for him, but his character and personality are recognizable again. It has been a great relief for him to know that his problems are physical, and not the result of sinfulness, or lack of faith. We have tried to comfort him, and our other children, with the reassurance that the Lord does not choose to heal all of us. There remain among us, righteous individuals that are still blind, still paraplegic. Severe anxiety/depressive disorder is like those other physical challenges. It is perhaps his lot in life. We all have our particular challenges to carry in this mortal life, and that is perhaps one of his, one that he must learn to cope with, as he endures to the end.
He still requires medication in order to be functional and happy, but he has regained some testimony that his Father in Heaven, and his Lord, Jesus Christ, have not forsaken him. His best moments come and go, but he is able to feel, spiritually, during his best moments.