I've worked at several startups now and I have never worked a full 8 hours per day. There's always some internet surfing, phone calls (which might or might not count as "work"), meetings (also might not count), lunch, coffee or smoke breaks, and general BS'ing with co-workers. It usually works out to 3-4 hours per day at max. Any experienced programming project manager knows that a "work day" in terms of coding is never more than 4 hours.
With that being said, I have at times taken time off to work on my own projects and forced myself to work a full 8 hours. No stopping of any kind, phone and instant messenger off, email closed, no breaks of any kind. It was incredibly difficult. I remember I would choose a playlist with 8 hours of music and not stop until it was over. But it was something I still do only rarely. It was very productive, however, but not something anyone could do every day.
With that being said, I have at times taken time off to work on my own projects and forced myself to work a full 8 hours. No stopping of any kind, phone and instant messenger off, email closed, no breaks of any kind. It was incredibly difficult. I remember I would choose a playlist with 8 hours of music and not stop until it was over. But it was something I still do only rarely. It was very productive, however, but not something anyone could do every day.