I'm a first-year Ph.D. student. I attempt to "work" a 40-hour week, but I count classes and classwork as part of that 40 hours and try not to count unproductive hours. Toward this end, instead of taking a full weekend off, I often take Thursday through Sunday "half off". I'm not sure whether this results in more or less work getting done than would otherwise. During my first semester, I felt very unproductive, but I've been fairly successful at getting things done this semester.