I go brain-dead when I feel I'm being useless, or when I feel I'm stagnating intellectually. Best thing to do is address the root causes. I've been applying my field in some of the most impoverished, vulnerable populations in the world, and although I am usually exhausted/look like crap, it's the best feeling in the world. To use your skills/knowledge to help those who really need it (and by default usually can't afford it): that's the best rush in life. Try it. If not in the UK, somewhere else.