The main "duty" of most non-tenured professors is to produce research. If you do that best by working regular 9am-5pm hours or by only coming in in the middle of the night, nobody's going to care much. Aside from that, you need to attend occasional meetings and turn your grades in at the end of the semester. Once you have tenure, the obligation to produce continuous research is lessened a bit, and most of the schedule on which you "fulfill your duties" is really up to you.
From my perspective in the trenches, the reduction is not as big as most people might think for CS and the sciences. If you worked like crazy while building your credentials, either for tenure or to a senior position in a non-tenure research track, you can't really slack off too much. You still need to bring in the cash to cover your team, grad student tuitions, and your own salary, which are now more expensive too. This means just as much research effort and proposal writing. This is exacerbated when research funding is cut at a large scale (sequestration). The reduction really comes from i) having established robust lab practices, methods, and management skills and ii) improved proposal writing skills combined with a track record. Junior faculty expend a lot of time finding and developing the right models, processes, and skills.
Another problem is that you spend your early career developing and reinforcing workaholic habits. It is very hard to step away from work, even for a regular weekend. Unlike most high intensity jobs, the flexible time is great for scheduling around family so they actually see you. You can insulate them from the worst of it.