In my opinion startups are about speed. That doesn't mean you have to work 14 hour days 7 days a week. It does mean that if some obstacle clears at 5:05 pm on a Friday, then you do the work to get to the next obstacle, even if it means you miss dinner with the family.
But if you're ahead of the game on a Monday morning at 9am, then sleep in, or go to a movie, or whatever. Because the best way to go fast is to be enthusiastic about what you're doing.