I've worked at three different startups so far, and I think it all boils down to two things: how much you believe in the product, and what the culture is like.

I've worked at three different startups so far, and I think it all boils down to two things: how much you believe in the product, and what the culture is like. At my first startup the culture was really great (in the beginning, at least) and the product was really cool. I had more fun working in that office than I would if I had been going out every night after work instead, so the 12 hour days didn't seem intrusive. At my second startup, it was the complete opposite. The culture sucked, and the product sucked. Which is why I put in what you're calling "bank hours" for a year and then left. Lots of the people there worked much, much longer days than I, and it's because they either believed in the product or they had responsibilities outside of work keeping them there (wife, kids, etc). Just a few anecdotal observations that (so far) held true for all the places I've worked.