Closing time.

War of Art by Steven Pressfield teaches us that a professional works on a schedule while an amateur works when he feels like it. The point that Pressfield is making is that the professional starts at the same time each day.

However, it may be even more important as a professional to end at the same time each day. To become aware of when you are closing the office and not to think about or do any work once you do that.

It's easier said than done, but building this skill is critical to sustain our work in the long run. The more specific we become about our work hours, the better work we will be able to do.