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.
Unless there's an external factor that requires that you complete a task by a certain day, it's much healthier to think of a startup as a marathon than series of sprints.
This may be true of YOUR startup. There are plenty of startups out there who have different thoughts about speed. And, so long as expectations are properly set with everyone on the team: co-founders, investors, et al, then whatever you need to do is the right thing regardless of what other startup founders around you are doing.
That depends too - are you going to miss some opportunity because you finished your work at 11:00 PM rather than 10:00 PM? Because if you're not, why not take an hour out, have dinner with the family, and then go back to work for a few hours once the kids are in bed?
I have several people on my team that arrange their day like that, and if I had a family, I would probably do it too. Actually, I often do it anyway, ducking out of work for a late afternoon or evening social activity and then putting in a couple hours from home around midnight. I find that taking a break often makes that late-night work session a lot more productive than if I'd just worked straight through.
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.