I am currently enrolled in a social science (management/information systems) PhD program, and I have spent a lot of time thinking/experimenting with how to maximize my performance. First I tried working 80 hours a week - this led to a hard time focusing, more time spent doing support activities (e.g. reading) rather than core activities (e.g. writing, coding etc.). Now I'm starting to settle more towards a 40-50 hour week that keeps me more energized the hours that I actually work.
I'm wondering, in your experience, what is the optimal balance of work/rest for graduate academic performance in the social sciences? Other factors that stimulate your academic performance?
I too have struggled at times to find this balance. I'm coming from math and computer science (rather than social science), but I think the field matters less than the type of work. If what you're trying to do is cognitively intense, then it's really important to be well-rested and to maintain focus. In grad school, I often would bang my head for hours on problems without much progress. Now if I work for 45 minutes or an hour with little progress, I'll go for a short walk, get a light snack, do jumping jacks, or play a quick web game; all of these are ways to reset my focus and attention.
Probably the single most useful skill I've learned is to be more aware of my current state (energy level, focus, sleepiness, hunger, emotional state) and try to choose work appropriate for (doable in) that state. For more thoughts on this idea, I recommend the advice given by Terry Tao here: http://terrytao.wordpress.com/2008/08/07/on-time-management/