It sounds like you got burnt out. When you "code until the sun comes up" you neglect basic human necessities like sleep and human contact. When you think only about the company and the product vision, you lose your sense of self. Your sense of self becomes your work.

I used to work at a startup where this was the case, but I've since changed. I love my work and my co-workers and I think they all work extremely hard, but I go home every night at 6:30. I cook for myself. I take salsa lessons. I read. And I work on my own projects.

Interesting work at companies comes with ebb and flows. Sometimes you work on things you don't like, sometimes you get bored, but if you keep your mind fresh and sane you'll come up with things to work on that are meaningful and interesting.