I read an interesting story by Benedict Carey of the New York Times.

Using  presidential papers and biographies three Doctors from Duke University Medical Center determined that almost half of the presidents between 1789 and 1974 suffered from a mental illness at some point in his life.
As a young man Abraham Lincoln had bouts of despair so profound that his friends thought he might commit suicide.
Grant often avoided social occasions and turned to drink. Sounds a lot like me.
Teddy Roosevelt and Lyndon Johnson showed symptoms of manic energy typical of bipolar disorder.
William H. Taft seems to have had sleep apnea.
Calvin Coolidge went into deep depression after his teenage son died of an infection.
Nixon drank heavily during the Watergate scandal.

They say that the percentage of presidents suffering mental illness matches the national average. They also said that it was encouraging that people suffering from mental illness can still function at a presidential level, if not at their best.
They also caution against judging troubled people too early, Madison, Hayes, Grant and Wilson appeared to do very little with their lives when they were younger.

They didn’t talk about any presidents after Nixon, but I would venture a guess that it may have raised the percentage.
I guess they really are average Americans.