I'm at a YC startup, and our culture is very much to not have late nights, work people to death, etc. The belief here is that working people to the bone does not build a scalable business, and the only way to grow at a sustainable pace is to maintain regular work hours, have realistic goals in terms of work, and to give people a life outside of work. Sure, we all monitor our emails on our phones, and on occasion we need to work a little extra, this is still Silicon Valley. But many people here have young families, and the office is pretty empty by 6:00pm.