Your work is your hobby. There is no difference (with all the problems that could originate from this, with respect to family life, etc.). In generaly, you enjoy so much what you are doing that you don't really care about many things out-of-academia people do. For instance, you are working 70 hours a week with being paid 35 hours a week. Your salary is probably miserable relatively to the level of education and to what you could get in the private sector, etc.

But you don't care. You like what you do, you want to understand better how the world works and possibly bring something to the mankind. Maybe this will happen, probably it won't :)