I think I know why the Ph.D is popularly known as "permanent head damage". Because if you spend THAT much time doing research and writing your thesis, you are bound to end up with head damage! Research and writing are generally solitary pursuits. You're going to end up living in your head! And as we all know, that ain't exactly healthy...
I love what Sir Ken Robinson says about education: that the more we have of it, the more it seems to focus mainly on what we have in our heads, and if you do things right, if you rise up to the pinnacle, you end up becoming an academic -- a university professor.
"There's something curious about professors -- in my experience, not all of them, but typically -- they live in their heads. They live up there, and slightly to one side. They're disembodied, you know, in a kind of literal way. They look upon their body as a form of transport for their heads. It's a way of getting their head to meetings." I love his humour.
So, yes. Working on my Master's thesis I already feel like I am getting head damage. I don't exactly want to make it permanent!