From my personal experience, I've found that doing physical exercise (paradoxically enough) seems to give me more energy. On those days where I've sat still all day I'm much more likely to feel like doing nothing after work, whereas when I've done my usual cycle commute I'm usually a lot more mentally alert. YMMV of course, but given the choice I'll personally rather do more than less physical exercise, even if it's small activities here and there like lugging some groceries.