There's a 15 page article in the NY times about this.
Somewhere they mentioned that Darwin was depressed for 2 out of every 3 days.
But he took his depression and asserted it into his work.

There are times when I am comfortable with the depression I'm in. I don't... enjoy it, per se, but I accept that it makes me think way more critically than most other people. I see scenarios that most people don't....

However, most of the time, I just hate it.