Also, I have my own anecdote.
I worked my tail off for a year in China. I had two other founders/partners back in N. America -- I was the only one on-the-ground, and I was also the youngest & most inexperienced.
I basically stayed there until I was physically, mentally, and emotionally sick. I was living in my office and had no salary (expenses covered, nothing more), a tough schedule (mornings & evenings needed to get back in touch w/ N. America), my first experience in management (6 staff or so), and had almost zero encouragement from my partners, because our expenses only increased with time as we realized what we were really getting in to (China is not exactly WYSIWYG when it comes to business) -- and I had no contact with the main decision-maker, who was the only person with any real experience in what we were doing (for political reasons I had to go through founder #2 to get to founder-investor#1).
I was literally stressed out of my mind. I lost a good portion of my short-term memory (until I quit). I could no longer predict what I was able to do in the future -- no ability to manage my own commitments & performance. Forgot why I wanted to live. Took up smoking.
I kept going as long as I did because I believed in the value of "sheer persistence". I wish I'd taken more drastic action sooner.
My experience was probably not typical, and so it's probably not worth drawing generalizations from it. But -- I'm incredibly glad that I quit. Today I'm learning new things and I'm happy to be alive -- I feel like a vibrant human being again. And I'm still excited about startups... but not to the point of self-destruction.